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Getting the first and last day of a month, using a given DateTime object

I want to get the first day and last day of the month where a given date lies in. The date comes from a value in a UI field.
If I'm using a time picker I could say
var maxDay = dtpAttendance.MaxDate.Day;
But I'm trying to get it from a DateTime object. So if I have this...
DateTime dt = DateTime.today;
How to get first day and last day of the month from dt?

DateTime structure stores only one value, not range of values. MinValue and MaxValue are static fields, which hold range of possible values for instances of DateTime structure. These fields are static and do not relate to particular instance of DateTime. They relate to DateTime type itself.
Suggested reading: static (C# Reference)
UPDATE: Getting month range:
DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);
UPDATE: From comments (#KarlGjertsen & #SergeyBerezovskiy)
DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddSeconds(-1);
//OR
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddTicks(-1);

This is more a long comment on #Sergey and #Steffen's answers. Having written similar code myself in the past I decided to check what was most performant while remembering that clarity is important too.
Result
Here is an example test run result for 10 million iterations:
2257 ms for FirstDayOfMonth_AddMethod()
2406 ms for FirstDayOfMonth_NewMethod()
6342 ms for LastDayOfMonth_AddMethod()
4037 ms for LastDayOfMonth_AddMethodWithDaysInMonth()
4160 ms for LastDayOfMonth_NewMethod()
4212 ms for LastDayOfMonth_NewMethodWithReuseOfExtMethod()
2491 ms for LastDayOfMonth_SpecialCase()
Code
I used LINQPad 4 (in C# Program mode) to run the tests with compiler optimization turned on. Here is the tested code factored as Extension methods for clarity and convenience:
public static class DateTimeDayOfMonthExtensions
{
public static DateTime FirstDayOfMonth_AddMethod(this DateTime value)
{
return value.Date.AddDays(1 - value.Day);
}
public static DateTime FirstDayOfMonth_NewMethod(this DateTime value)
{
return new DateTime(value.Year, value.Month, 1);
}
public static DateTime LastDayOfMonth_AddMethod(this DateTime value)
{
return value.FirstDayOfMonth_AddMethod().AddMonths(1).AddDays(-1);
}
public static DateTime LastDayOfMonth_AddMethodWithDaysInMonth(this DateTime value)
{
return value.Date.AddDays(DateTime.DaysInMonth(value.Year, value.Month) - value.Day);
}
public static DateTime LastDayOfMonth_SpecialCase(this DateTime value)
{
return value.AddDays(DateTime.DaysInMonth(value.Year, value.Month) - 1);
}
public static int DaysInMonth(this DateTime value)
{
return DateTime.DaysInMonth(value.Year, value.Month);
}
public static DateTime LastDayOfMonth_NewMethod(this DateTime value)
{
return new DateTime(value.Year, value.Month, DateTime.DaysInMonth(value.Year, value.Month));
}
public static DateTime LastDayOfMonth_NewMethodWithReuseOfExtMethod(this DateTime value)
{
return new DateTime(value.Year, value.Month, value.DaysInMonth());
}
}
void Main()
{
Random rnd = new Random();
DateTime[] sampleData = new DateTime[10000000];
for(int i = 0; i < sampleData.Length; i++) {
sampleData[i] = new DateTime(1970, 1, 1).AddDays(rnd.Next(0, 365 * 50));
}
GC.Collect();
System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].FirstDayOfMonth_AddMethod();
}
string.Format("{0} ms for FirstDayOfMonth_AddMethod()", sw.ElapsedMilliseconds).Dump();
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].FirstDayOfMonth_NewMethod();
}
string.Format("{0} ms for FirstDayOfMonth_NewMethod()", sw.ElapsedMilliseconds).Dump();
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].LastDayOfMonth_AddMethod();
}
string.Format("{0} ms for LastDayOfMonth_AddMethod()", sw.ElapsedMilliseconds).Dump();
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].LastDayOfMonth_AddMethodWithDaysInMonth();
}
string.Format("{0} ms for LastDayOfMonth_AddMethodWithDaysInMonth()", sw.ElapsedMilliseconds).Dump();
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].LastDayOfMonth_NewMethod();
}
string.Format("{0} ms for LastDayOfMonth_NewMethod()", sw.ElapsedMilliseconds).Dump();
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].LastDayOfMonth_NewMethodWithReuseOfExtMethod();
}
string.Format("{0} ms for LastDayOfMonth_NewMethodWithReuseOfExtMethod()", sw.ElapsedMilliseconds).Dump();
for(int i = 0; i < sampleData.Length; i++) {
sampleData[i] = sampleData[i].FirstDayOfMonth_AddMethod();
}
GC.Collect();
sw.Restart();
for(int i = 0; i < sampleData.Length; i++) {
DateTime test = sampleData[i].LastDayOfMonth_SpecialCase();
}
string.Format("{0} ms for LastDayOfMonth_SpecialCase()", sw.ElapsedMilliseconds).Dump();
}
Analysis
I was surprised by some of these results.
Although there is not much in it the FirstDayOfMonth_AddMethod was slightly faster than FirstDayOfMonth_NewMethod in most runs of the test. However, I think the latter has a slightly clearer intent and so I have a preference for that.
LastDayOfMonth_AddMethod was a clear loser against LastDayOfMonth_AddMethodWithDaysInMonth, LastDayOfMonth_NewMethod and LastDayOfMonth_NewMethodWithReuseOfExtMethod. Between the fastest three there is nothing much in it and so it comes down to your personal preference. I choose the clarity of LastDayOfMonth_NewMethodWithReuseOfExtMethod with its reuse of another useful extension method. IMHO its intent is clearer and I am willing to accept the small performance cost.
LastDayOfMonth_SpecialCase assumes you are providing the first of the month in the special case where you may have already calculated that date and it uses the add method with DateTime.DaysInMonth to get the result. This is faster than the other versions, as you would expect, but unless you are in a desperate need for speed I don't see the point of having this special case in your arsenal.
Conclusion
Here is an extension method class with my choices and in general agreement with #Steffen I believe:
public static class DateTimeDayOfMonthExtensions
{
public static DateTime FirstDayOfMonth(this DateTime value)
{
return new DateTime(value.Year, value.Month, 1);
}
public static int DaysInMonth(this DateTime value)
{
return DateTime.DaysInMonth(value.Year, value.Month);
}
public static DateTime LastDayOfMonth(this DateTime value)
{
return new DateTime(value.Year, value.Month, value.DaysInMonth());
}
}
If you have got this far, thank you for time! Its been fun :¬). Please comment if you have any other suggestions for these algorithms.

Getting month range with .Net API (just another way):
DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = new DateTime(date.Year, date.Month, DateTime.DaysInMonth(date.Year, date.Month));

"Last day of month" is actually "First day of *next* month, minus 1". So here's what I use, no need for "DaysInMonth" method:
public static DateTime FirstDayOfMonth(this DateTime value)
{
return new DateTime(value.Year, value.Month, 1);
}
public static DateTime LastDayOfMonth(this DateTime value)
{
return value.FirstDayOfMonth()
.AddMonths(1)
.AddMinutes(-1);
}
NOTE:
The reason I use AddMinutes(-1), not AddDays(-1) here is because usually you need these date functions for reporting for some date-period, and when you build a report for a period, the "end date" should actually be something like Oct 31 2015 23:59:59 so your report works correctly - including all the data from last day of month.
I.e. you actually get the "last moment of the month" here. Not Last day.
OK, I'm going to shut up now.

DateTime dCalcDate = DateTime.Now;
dtpFromEffDate.Value = new DateTime(dCalcDate.Year, dCalcDate.Month, 1);
dptToEffDate.Value = new DateTime(dCalcDate.Year, dCalcDate.Month, DateTime.DaysInMonth(dCalcDate.Year, dCalcDate.Month));

Here you can add one month for the first day of current month than delete 1 day from that day.
DateTime now = DateTime.Now;
var startDate = new DateTime(now.Year, now.Month, 1);
var endDate = startDate.AddMonths(1).AddDays(-1);

If you only care about the date
var firstDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind).AddMonths(1).AddDays(-1);
If you want to preserve time
var firstDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind).AddMonths(1).AddDays(-1);

Try this one:
string strDate = DateTime.Now.ToString("MM/01/yyyy");

The accepted answer here does not take into account the Kind of the DateTime instance. For example if your original DateTime instance was a UTC Kind then by making a new DateTime instance you will be making an Unknown Kind instance which will then be treated as local time based on server settings. Therefore the more proper way to get the first and last date of the month would be this:
var now = DateTime.UtcNow;
var first = now.Date.AddDays(-(now.Date.Day - 1));
var last = first.AddMonths(1).AddTicks(-1);
This way the original Kind of the DateTime instance is preserved.

I used this in my script(works for me) but I needed a full date without the need of trimming it to only the date and no time.
public DateTime GetLastDayOfTheMonth()
{
int daysFromNow = DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month) - (int)DateTime.Now.Day;
return DateTime.Now.AddDays(daysFromNow);
}

For Persian culture
PersianCalendar pc = new PersianCalendar();
var today = pc.GetDayOfMonth(DateTime.Now);
var firstDayOfMonth = pc.GetDayOfMonth(DateTime.Now.AddDays(-(today-1)));
var lastDayOfMonth = pc.GetDayOfMonth(DateTime.Now.AddMonths(1).AddDays(-today));
Console.WriteLine("First day "+ firstDayOfMonth);
Console.WriteLine("Last day " + lastDayOfMonth);

You can do it
DateTime dt = DateTime.Now;
DateTime firstDayOfMonth = new DateTime(dt.Year, date.Month, 1);
DateTime lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);

Give this a try. It basically calculates the number of days that has passed on DateTime.Now, then subtracts one from that and uses the new value to find the first of the current month. From there it uses that DateTime and uses .AddMonths(-1) to get the first of the previous month.
Getting the last day of last month does basically the same thing except it adds one to number of days in the month and subtracts that value from DateTime.Now.AddDays, giving you the last day of the previous month.
int NumberofDays = DateTime.Now.Day;
int FirstDay = NumberofDays - 1;
int LastDay = NumberofDays + 1;
DateTime FirstofThisMonth = DateTime.Now.AddDays(-FirstDay);
DateTime LastDayOfLastMonth = DateTime.Now.AddDays(-LastDay);
DateTime CheckLastMonth = FirstofThisMonth.AddMonths(-1);

You can try this for get current month first day;
DateTime.Now.AddDays(-(DateTime.Now.Day-1))
and assign it a value.
Like this:
dateEndEdit.EditValue = DateTime.Now;
dateStartEdit.EditValue = DateTime.Now.AddDays(-(DateTime.Now.Day-1));

Create an instance of DateTime class
DateTime dateTime = DateTime.Now;
If you want to get the last day of the month you can do this
int lastDayOfMonth = DateTime.DaysInMonth(caducidadPuntos.Year, caducidadPuntos.Month);
If you want to get the first day of the month, you can do this
DateTime firstDayMonth = new DateTime(dateTime.Year, dateTime.Month, 1);

We had the requirement of being able to get the start and end of a given dates month, including times, inclusively. We ended up utilizing the aforementioned solutions, huge thanks to everyone here, and combined it into a util class to be able to get the start and end for a given month and year number combination up to the last millisecond. Including what we moved forward with in the event it helps someone else.
The util:
public class DateUtil
{
public static (DateTime startOfMonth, DateTime endOfMonth) GetStartAndEndOfMonth(int month, int year)
{
DateTime startOfMonth = GetStartOfMonth(month, year);
DateTime endOfMonth = GetEndOfMonth(month, year);
return (startOfMonth, endOfMonth);
}
public static DateTime GetStartOfMonth(int month, int year)
{
return new DateTime(year, month, 1).Date;
}
public static DateTime GetEndOfMonth(int month, int year)
{
return new DateTime(year, month, 1).Date.AddMonths(1).AddMilliseconds(-1);
}
}
Usage:
(DateTime startOfMonth, DateTime endOfMonth) = DateUtil.GetStartAndEndOfMonth(2, 2021); // February, 2021

easy way to do it
Begin = new DateTime(DateTime.Now.Year, DateTime.Now.Month,1).ToShortDateString();
End = new DataFim.Text = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month)).ToShortDateString();

DateTime dCalcDate = DateTime.Now;
var startDate = new DateTime(Convert.ToInt32(Year), Convert.ToInt32(Month), 1);
var endDate = new DateTime(Convert.ToInt32(Year), Convert.ToInt32(Month), DateTime.DaysInMonth((Convert.ToInt32(Year)), Convert.ToInt32(Month)));

Time difference in C#, 3-digit-format minutes

I have a problem calculating difference between two timestamps, when minutes are represented with 3 digits, e.g. 180:22 = 180 minutes and 22 seconds.
So, can you help me how can I get difference between timestamps like:
180:22 and 122:11
or
232:21 and 31:34
etc.
UPDATE: I need to get difference between two times, defined as strings. What makes a problem is that minutes in those strings (times) are larger than 60, and they are over the limit. So I need to know how to find difference like in above examples (180:22 and 122:11, and 232:21 and 31:34)

Use System.TimeSpan structures:
var seconds=(new TimeSpan(0, 180, 22)-new TimeSpan(0, 122, 11)).TotalSeconds;
var minutes=(new TimeSpan(0, 232, 21)-new TimeSpan(0, 31, 34)).TotalMinutes;

Here's a class that will do this stuff:
public class CrazyTime
{
public TimeSpan TimeSpanRepresentation { get; set; }
public CrazyTime(TimeSpan timeSpan)
{
this.TimeSpanRepresentation = timeSpan;
}
public CrazyTime(string crazyTime)
{
// No error checking. Add if so desired
var pieces = crazyTime.Split(new[] { ':' });
var minutes = int.Parse(pieces[0]);
var seconds = int.Parse(pieces[1]);
TimeSpanRepresentation = new TimeSpan(0, minutes, seconds);
}
public static CrazyTime operator-(CrazyTime left, CrazyTime right)
{
var newValue = left.TimeSpanRepresentation - right.TimeSpanRepresentation;
return new CrazyTime(newValue);
}
public override string ToString()
{
// How should negative Values look?
return ((int)Math.Floor(TimeSpanRepresentation.TotalMinutes)).ToString() + ":" + TimeSpanRepresentation.Seconds.ToString();
}
}
Here's how it might be used:
var a = new CrazyTime("123:22");
var b = new CrazyTime("222:11");
var c = b - a;
Console.WriteLine(c);

This works:
string time1 = "180:22";
string time2 = "122:11";
TimeSpan span1 = getTimespan(time1);
TimeSpan span2 = getTimespan(time2);
TimeSpan diff = span1 - span2;
getTimespan just has to correctly parse the string. I decided on Regex to do that, but you could go any route, particularly if the delimiter ":" isn't ever going to change.
private static TimeSpan getTimespan(string time1)
{
Regex reg = new Regex(#"\d+");
MatchCollection matches = reg.Matches(time1);
if (matches.Count == 2)
{
int minutes = int.Parse(matches[0].Value);
int seconds = int.Parse(matches[1].Value);
return new TimeSpan(0, minutes, seconds);
}
return TimeSpan.Zero;
}

How to convert Milliseconds to date format in C#?

In C# how can I convert Unix-style timestamp to yyyy-MM-ddThh:mm:ssZ?

Start by converting your milliseconds to a TimeSpan:
var time = TimeSpan.FromMilliseconds(milliseconds);
Now, in .NET 4 you can call .ToString() with a format string argument. See http://msdn.microsoft.com/en-us/library/system.timespan.tostring.aspx
In previous versions of .NET, you'll have to manually construct the formatted string from the TimeSpan's properties.

new DateTime(numTicks * 10000)
The DateTime(long ticks) constructor is what you need. Each tick represents 100 nanoseconds so multiply by 10000 to get to 1 millisecond.

If the milliseconds is based on UNIX epoch time, then you can use:
var posixTime = DateTime.SpecifyKind(new DateTime(1970, 1, 1), DateTimeKind.Utc);
var time = posixTime.AddMilliseconds(milliSecs);

This worked for me:
DateTimeOffset.FromUnixTimeMilliseconds(milliseconds);
You can get just the DateTime from that if you need it.

Here you go:
public static class UnixDateTime
{
public static DateTimeOffset FromUnixTimeSeconds(long seconds)
{
if (seconds < -62135596800L || seconds > 253402300799L)
throw new ArgumentOutOfRangeException("seconds", seconds, "");
return new DateTimeOffset(seconds * 10000000L + 621355968000000000L, TimeSpan.Zero);
}
public static DateTimeOffset FromUnixTimeMilliseconds(long milliseconds)
{
if (milliseconds < -62135596800000L || milliseconds > 253402300799999L)
throw new ArgumentOutOfRangeException("milliseconds", milliseconds, "");
return new DateTimeOffset(milliseconds * 10000L + 621355968000000000L, TimeSpan.Zero);
}
public static long ToUnixTimeSeconds(this DateTimeOffset utcDateTime)
{
return utcDateTime.Ticks / 10000000L - 62135596800L;
}
public static long ToUnixTimeMilliseconds(this DateTimeOffset utcDateTime)
{
return utcDateTime.Ticks / 10000L - 62135596800000L;
}
[Test]
public void UnixSeconds()
{
DateTime utcNow = DateTime.UtcNow;
DateTimeOffset utcNowOffset = new DateTimeOffset(utcNow);
long unixTimestampInSeconds = utcNowOffset.ToUnixTimeSeconds();
DateTimeOffset utcNowOffsetTest = UnixDateTime.FromUnixTimeSeconds(unixTimestampInSeconds);
Assert.AreEqual(utcNowOffset.Year, utcNowOffsetTest.Year);
Assert.AreEqual(utcNowOffset.Month, utcNowOffsetTest.Month);
Assert.AreEqual(utcNowOffset.Date, utcNowOffsetTest.Date);
Assert.AreEqual(utcNowOffset.Hour, utcNowOffsetTest.Hour);
Assert.AreEqual(utcNowOffset.Minute, utcNowOffsetTest.Minute);
Assert.AreEqual(utcNowOffset.Second, utcNowOffsetTest.Second);
}
[Test]
public void UnixMilliseconds()
{
DateTime utcNow = DateTime.UtcNow;
DateTimeOffset utcNowOffset = new DateTimeOffset(utcNow);
long unixTimestampInMilliseconds = utcNowOffset.ToUnixTimeMilliseconds();
DateTimeOffset utcNowOffsetTest = UnixDateTime.FromUnixTimeMilliseconds(unixTimestampInMilliseconds);
Assert.AreEqual(utcNowOffset.Year, utcNowOffsetTest.Year);
Assert.AreEqual(utcNowOffset.Month, utcNowOffsetTest.Month);
Assert.AreEqual(utcNowOffset.Date, utcNowOffsetTest.Date);
Assert.AreEqual(utcNowOffset.Hour, utcNowOffsetTest.Hour);
Assert.AreEqual(utcNowOffset.Minute, utcNowOffsetTest.Minute);
Assert.AreEqual(utcNowOffset.Second, utcNowOffsetTest.Second);
Assert.AreEqual(utcNowOffset.Millisecond, utcNowOffsetTest.Millisecond);
}
}

This sample will demonstrate the general idea, but you need to know if your starting date is DateTime.MinValue or something else:
int ms = 1000; // One second
var date = new DateTime(ms * 10000); // The constructor takes number of 100-nanoseconds ticks since DateTime.MinValue (midnight, january 1st, year 1)
string formatted = date.ToString("yyyy-MM-ddTHH:mm:ssZ");
Console.WriteLine(formatted);

You can construct your datetime from ticks:
long ticks = new DateTime(1979, 07, 28, 22, 35, 5,
new CultureInfo("en-US", false).Calendar).Ticks;
DateTime dt3 = new DateTime(ticks);
Console.Write(dt3.ToString("yyyy-MM-ddThh:mm:ssZ"));

private static DateTime Milliseconds2Date(Double d)
{
TimeSpan time = TimeSpan.FromMilliseconds(d);
return new DateTime(1970, 1, 1) + time;
}
private static Double Date2Milliseconds(DateTime d)
{
var t = d.Subtract(new DateTime(1970, 1, 1));
return t.TotalMilliseconds;
}

This question should have the answer you need.
Short version:
DateTime date = new DateTime(long.Parse(ticks));
date.ToString("yyyy-MM-ddThh:mm:ssZ");

Elegantly check if a given date is yesterday

Assuming you have a Unix timestamp, what would be an easy and/or elegant way to check if that timestamp was some time yesterday?
I am mostly looking for solutions in Javascript, PHP or C#, but pseudo code and language agnostic solutions (if any) are welcome as well.

In C# you could use this:
bool isYesterday = DateTime.Today - time.Date == TimeSpan.FromDays(1);

You can use this in C#:
bool isYesterday = (dateToCheck.Date.AddDays(1) == DateTime.Now.Date);

PHP:
$isYesterday = date('Ymd', $timestamp) == date('Ymd', strtotime('yesterday'));

In pseudo code, to compare timestamps:
get current Unix timestamp
transform the retrieved timestamp to a date
subtract 1 day from the date
transform the timestamp to test to a date
compare both dates. If they're equal the tested timestamp was yesterday.
Watch out for timezones if you show the results to a user. For me it's now 13:39 on July 9 2010. A timestamp for 14 hours ago for me is yesterday. But for someone in a different timezone where it's now 15:39, 14 hours ago wasn't yesterday!
Another problem might be systems with a wrong time/date setup. For example if you use JavaScript and the system time of the visitors PC is wrong, the program may come to a wrong conclusion. If it's essential to get a correct answer, retrieve the current time from a known source with a correct time.

An example in Smalltalk using Pharo/Squeak
(Date year: 2014 month: 4 day: 24) = Date yesterday

This accepts an optional DateTimeZone object. If it's not given, it uses the currently set default timezone.
<?php
function isYesterday($timestamp, $timezone = null) {
$t = new DateTime(null, $timezone);
$t->setTimestamp($timestamp);
$t->setTime(0,0);
$yesterday = new DateTime("now", $timezone);
$yesterday->setTime(0,0);
$yesterday = $yesterday->sub(new DateInterval('P1D'));
return $t == $yesterday;
}

Another C# example:
bool isYesterday = DateTime.Now.Date.AddDays(-1) == dateToCheck.Date;

Code:
static class ExtensionMethods
{
private static readonly DateTime UnixStart = new DateTime(1970, 1, 1, 0, 0, 0, 0);;
public static bool IsYesterday(this int unixTime)
{
DateTime convertedTime = UnixStart.AddSeconds(unixTime);
return convertedTime.Date == DateTime.Now.AddDays(-1).Date;
}
public static bool IsYesterday(this DateTime date)
{
return date.Date == DateTime.Now.AddDays(-1).Date;
}
}
Examples:
public class Examples
{
public void Tests()
{
if (1278677571.IsYesterday()) System.Console.WriteLine("Is yesterday");
DateTime aDate = new DateTime(2010, 12, 31);
if (aDate.IsYesterday()) System.Console.WriteLine("Is yesterday");
}
}

In JavaScript, you could write
var someDate = new Date(2010, 6, 9);
Date.yesterday.date == someDate.date // true
Left out needless implementation details, but it's possible. Ok, there ya go :)
(function() {
function date(d) {
var year = d.getFullYear();
var month = d.getMonth();
var day = d.getDate();
return new Date(year, month, day);
}
Object.defineProperty(Date, 'yesterday', {
enumerable: true,
configurable: false,
get: function() {
var today = new Date();
var millisecondsInADay = 86400000;
var yesterday = new Date(today - millisecondsInADay);
return yesterday;
},
set: undefined
});​​​​​​​​
Object.defineProperty(Date.prototype, 'date', {
enumerable: true,
configurable: true,
get: function() {
return date(this).valueOf();
},
set: undefined
});
})();

C#
TimeSpan difference = DateTime.Now.Date - olderDate.Date;
bool isYesterday = difference.TotalDays == 1;

You can give this function a shot:
public bool IsFromYesterday(long unixTime) {
DateTime convertedTime = new DateTime(1970, 1, 1, 0, 0, 0, 0);
convertedTime.AddSeconds(unixTime);
DateTime rightNow = DateTime.Now;
DateTime startOfToday = DateTime.Today;
DateTime startOfYesterday = startOfToday - new TimeSpan(1, 0, 0, 0);
if (convertedTime > startOfYesterday && convertedTime < rightNow)
return true;
else
return false;
}

reverse this function

i have code that takes a csharp datetime and converts it into a long to plot in the "flot" graph. here is the code
public static long GetJavascriptTimestamp(DateTime input)
{
TimeSpan span = new TimeSpan(DateTime.Parse("1/1/1970").Ticks);
DateTime time = input.Subtract(span);
return (long)(time.Ticks / 10000);
}
I now need an opposite function where i take this long value and get the csharp datetime object back. any idea if the above method can be reversed ?

DateTime date = new DateTime(1970, 1, 1).Add(new TimeSpan(yourLong * 10000));

Aren't you just looking for this?
public static DateTime DateTimeFromJavascript(long millisecs)
{
return new DateTime(1970, 1, 1).AddMilliseconds(millisecs);
}

Can be:
public static DateTime GetTimestampFromJS(long ts)
{
DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
return origin.AddSeconds(ts*1000);
}