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C# - Validate an int-based DateTime without exceptions?

This question talks about validating a string representing a date, and in it folks mention that it's good to avoid using Exceptions for regular flow logic. And TryParse() is great for that. But TryParse() takes a string, and in in my case i've already got the year month and day as integers. I want to validate the month/day/year combination. For example February 30th.
It's pretty easy to just put a try/catch around new DateTime(int, int, int), but I'm wondering if there's a way to do it without relying on exceptions.
I'd also feel silly composing these ints into a string and then using TryParse().

The following will check for valid year/month/day combinations in the range supported by DateTime, using a proleptic Gregorian calendar:
public bool IsValidDate(int year, int month, int day)
{
return year >= 1 && year <= 9999
&& month >= 1 && month <= 12
&& day >= 1 && day <= DateTime.DaysInMonth(year, month);
}
If you need to work with other calendar systems, then expand it as follows:
public bool IsValidDate(int year, int month, int day, Calendar cal)
{
return year >= cal.GetYear(cal.MinSupportedDateTime)
&& year <= cal.GetYear(cal.MaxSupportedDateTime)
&& month >= 1 && month <= cal.GetMonthsInYear(year)
&& day >= 1 && day <= cal.GetDaysInMonth(year, month);
}

Use String Interpolation
int year = 2017;
int month = 2;
int day = 28;
DateTime dt;
DateTime.TryParse($"{month}/{day}/{year}", out dt);

As far as I know, there's no easy way to check for a DateTime's int's validity besides concatenating the ints into a correctly formatted string beforehand.
To avoid try/catch-ing, I would write a static utility class which utilizes DateTime.TryParse:
using System;
public static class DateTimeUtilities
{
public static bool TryParse(int year, int month, int day, out DateTime result)
{
return DateTime.TryParse(
string.Format("{0}/{1}/{2}", year, month, day), out result);
}
}
Usage:
DateTime dateTime;
if (DateTimeUtilities.TryParse(2017, 2, 30, out dateTime))
{
// success
}
else
{
// fail, dateTime = DateTime.MinValue
}
Pending the needs of your application, e.g. culture (thanks #Matt Johnson), I would also look into DateTime.TryParseExact.

Look at it this way. Any code that you write:
Will have to check month ranges 1-12
Will have to check day ranges by month, which means you'll have to hard code an array
Will have to account for leap years, which can be a pain the the rear
Rather than doing ALL that, and reinventing the wheel, and potentially getting it wrong -- why don't you keep it simple and just wrap the DateTime constructor in a try-catch and keep it moving? Let the nerds up in Redmond do all the hard work for this common task. The best solution is one that any developer following you can understand and rely upon quickly.
I'd bet money that under the hood, TryParse and the DateTime constructor are using the exact same validators, except that the latter throws an exception while the former does not. TryParse, for this, is overkill with all the extra string manipulation involved.

How to compare a given date from today

I want to compare a given date to today and here is the condition: If provided date is greater than or equal to 6 months earlier from today, return true else return false
Code:
string strDate = tbDate.Text; //2015-03-29
if (DateTime.Now.AddMonths(-6) == DateTime.Parse(strDate)) //if given date is equal to exactly 6 months past from today (change == to > if date has to be less 6 months)
{
lblResult.Text = "true"; //this doesn't work with the entered date above.
}
else //otherwise give me the date which will be 6 months from a given date.
{
DateTime dt2 = Convert.ToDateTime(strDate);
lblResult.Text = "6 Months from given date is: " + dt2.AddMonths(6); //this works fine
}
If 6 months or greater than 6 months is what I would like for one
condition
If less than 6 months is another condition.

Your first problem is that you're using DateTime.Now instead of DateTime.Today - so subtracting 6 months will give you another DateTime with a particular time of day, which is very unlikely to be exactly the date/time you've parsed. For the rest of this post, I'm assuming that the value you parse is really a date, so you end up with a DateTime with a time-of-day of midnight. (Of course, in my very biased view, it would be better to use a library which supports "date" as a first class concept...)
The next problem is that you are assuming that subtracting 6 months from today and comparing it with a fixed date is equivalent to adding 6 months to the fixed date and comparing it with today. They're not the same operation - calendar arithmetic just doesn't work like that. You should work out which way you want it to work, and be consistent. For example:
DateTime start = DateTime.Parse(tbDate.Text);
DateTime end = start.AddMonths(6);
DateTime today = DateTime.Today;
if (end >= today)
{
// Today is 6 months or more from the start date
}
else
{
// ...
}
Or alternatively - and not equivalently:
DateTime target = DateTime.Parse(tbDate.Text);
DateTime today = DateTime.Today;
DateTime sixMonthsAgo = today.AddMonths(-6);
if (sixMonthsAgo >= target)
{
// Six months ago today was the target date or later
}
else
{
// ...
}
Note that you should only evaluate DateTime.Today (or DateTime.Now etc) once per set of calculations - otherwise you could find it changes between evaluations.

Try with this
DateTime s = Convert.ToDateTime(tbDate.Text);
s = s.Date;
if (DateTime.Today.AddMonths(-6) == s) //if given date is equal to exactly 6 months past from today (change == to > if date has to be less 6 months)
{
lblResult.Text = "true"; //this doesn't work with the entered date above.
}
replace == with >= or <= according to your needs

C# 12 hour time difference with integers

I have 14 textboxes that takes a user’s input of two times in a 24 hour clock format. When the calculate button is clicked the difference between the two times is calculated and returns the time in decimal format to the respective label. Ideally I would like the user to simply enter time as an integer, such as 1253 or 925 and select AM or PM from the drop down box. Say a user enters 1115 as the in time with AM selected then enters 300 as the out time with PM selected (as shown in the example entry below), the calculate button is clicked and 3.75 is returned in the label.
I have this code below and it works but I get errors when there aren’t exactly four characters. First question, how do I fix this so if an integer such as 800 will be read as 8:00 and not error out?
DateTime dt = DateTime.ParseExact(MondayW1InTextBox.Text, "HHmm", CultureInfo.InvariantCulture);
string timestring = dt.ToString("h:mm");
MondayW1Label.Text = timestring;
Second, once the string is formatted to 12 hour format, how can I get it to take the AM/PM drop down list as an argument for calculating the difference?
Below is the current C# code behind for just the Monday textboxes calculation which is just 24 hour time format, but want to move away from 24 hour time.
protected void CalculateButton_Click(object sender, EventArgs e)
{
TimeSpan TimeIn, TimeOut;
if (!TimeSpan.TryParse(MondayW1InTextBox.Text, out TimeIn)) TimeIn = default(TimeSpan);
if (!TimeSpan.TryParse(MondayW1OutTextBox.Text, out TimeOut)) TimeOut = default(TimeSpan);
MondayW1Label.Text = (TimeOut - TimeIn).TotalHours.ToString("f2");
}

Your first problem is related to the pattern you are using to parse the time: ParseExact will always try to match the exact pattern (in your case, "HHmm") to the string being parsed. That means it expects two digits representing the hours and two digits for the minutes. You can easily make it work if you append a leading zero to your string whenever its size is < 4. You can use the PadLeft method for doing that:
DateTime dt = DateTime.ParseExact(MondayW1InTextBox.Text.PadLeft(4, '0'), "HHmm", CultureInfo.InvariantCulture);
The first argument of PadLeft is the total length of the resulting string (in our case, 4), and the second argument is the character that should be used to fill in ('0').
For your second problem, you can parse the strings to get the DateTime object and, if the PM value is selected, just add 12 hours to the corresponding time.
DateTime timeIn = DateTime.ParseExact(MondayW1InTextBox.Text.PadLeft(4, '0'), "HHmm", CultureInfo.InvariantCulture);
DateTime timeOut = DateTime.ParseExact(MondayW1OutTextBox.Text.PadLeft(4, '0'), "HHmm", CultureInfo.InvariantCulture);
if(dropDownListIn.SelectedValue == "PM") timeIn = timeIn.AddHours(12);
if(dropDownListOut.SelectedValue == "PM") timeOut = timeOut.AddHours(12);
MondayW1Label.Text = (timeOut - timeIn).TotalHours.ToString("f2");
Notice that you can use the subtraction operator on DateTime objects to get the time difference between them, no need to explicitly convert them to TimeSpans.

Maybe this example (it is pretty crude but you get the logic) will be helpful:
string datetime1 = "800";
DateTime dt1 = DateTime.ParseExact((datetime1.Length == 3) ? "0" + datetime1 : datetime1, "hhmm", CultureInfo.InvariantCulture);
string dropDownVal = "AM";
if (dropDownVal == "PM")
dt1 = dt1.AddHours (12);
string datetime2 = "1100";
DateTime dt2 = DateTime.ParseExact((datetime2.Length == 3) ? "0" + datetime2 : datetime2, "hhmm", CultureInfo.InvariantCulture);
dropDownVal = "PM";
if (dropDownVal == "PM")
dt2 = dt2.AddHours (12);
TimeSpan TimeIn, TimeOut;
TimeIn = new TimeSpan (dt1.Ticks);
TimeOut = new TimeSpan(dt2.Ticks);
Console.WriteLine((TimeOut - TimeIn).TotalHours.ToString("f2"));
Console.ReadLine ( );

Date Comparing in C#

i'm currently working on a little project and i'm stuck with a little problem.
I would like my program to call a method CheckDate on boot.
This method would read in a .txt file to see the last saved date in (yyyy/mm/dd) format.
Then it would compare it with todays date and if it's not the same go on with some instructions.I've read the doc here but can't quite find which method best suites my need.
Question 1: Is there a way to get today's date in (yyyy/mm/dd) format?
Question 2: What's the easiest way to compare Dates in C#?
Thanks in advance.

1. DateTime.Now.ToString("yyyy/MM/dd")
2. DateTime.Parse(input).Date == DateTime.Now.Date

You can get today's date as a string by simply formatting a date.
String today = String.Format("{0: yyyy/MM/dd}", DateTime.Now);
String today = DateTime.Now.ToString("yyyy/MM/dd");
I would advise against using a text file as your means of saving data but if you are going with that system the only thing you would have to do is check to see if the date from the text file matches the date you formatted. Simply comparing formatted strings should do the trick.
if (string a == string b)
You could even put it in one line without having to format stuff separately
if (DateTime.Now.ToString("yyyy/MM/dd").Equals("date pulled from txt file"))

What's the easiest way to compare Dates in C#?
Store them not as text but in a DatteTime.
Compare the variables.
If there is a time in both, compare a.Date == b.Date.
Is there a way to get today's date in (yyyy/mm/dd) format?
Yes. This is wrong, though. PARSE The wrong input and compare the parsed data.

There is a DateTime.Compare method that you could use http://msdn.microsoft.com/en-us/library/system.datetime.compare.aspx - this should also let you use the built-in < and > operators.

By the letter of the question:
1:
DateTime.Now.ToString(#"yyyy\/MM\/dd")
2:
if(d1 < d2)...
if(d2 >= d1)...
etc.
However.
DateTime dt;
if(DateTime.TryParseExact(readInString, "yyyy-MM-dd", null, DateTimeStyles.AssumeLocal, out dt))
{
if(dt != DateTime.Now.Date)
{
//Code for case where it's no longer that day goes here.
}
}
else
{
//Code for someone messed up the file and it's not a valid date any more goes here.
}
You're doing this for computer-reading, not human-reading, so use the standard format rather than the conventional format (standard as in ISO, but also every country except North Korea has it as the national standard): yyyy-MM-dd (Edit: I see you're in Canada, CSA Z234.5:1989 is the relevant national standard on date-times for technical purposes; it says to use yyyy-MM-dd).
You should do it the other way around, read the string, parse the date, and do the comparison.

you might want to have a look at the FileInfo-Class ... you can compare the LastWriteTime Member to DateTime.Today

DateTime d1 = DateTime.Now;
DateTime d2 = d1.AddMilliseconds(123456789);
string formattedDate = d1.ToString("yyyy/MM/dd");
TimeSpan ts = d2 - d1;
double dayDiff = ts.TotalDays;
double hourDiff = ts.TotalHours;
double minuteDiff = ts.TotalMinutes;
double secondDiff = ts.TotalSeconds;
double milDiff = ts.TotalMilliseconds;
Console.WriteLine("Formatted Date: {0}\r\nDate Diff:\r\nTotal Days: {1}; Total Hours: {2}; Total Minutes: {3}; Total Seconds: {4}; Total Milliseconds: {5}", formattedDate,dayDiff,hourDiff,minuteDiff,secondDiff,milDiff);
Output:
Formatted Date: 2011/12/15
Date Diff:
Total Days: 1.42889802083333; Total Hours: 34.2935525; Total Minutes: 2057.61315; Total Seconds:
123456.789; Total Milliseconds: 123456789
*Edited my initial post to clarify how the "Total" properties work.

//use a TimeSpan do something like this
strCurDate = string.Format(DateTime.Now.ToString(), "yyyy/mm/dd");
FileInfo fiUpdateFileFile = null;
fiUpdateFileFile = new FileInfo(YourFile Location + Your FileName);
if (((TimeSpan)(DateTime.Now - fiUpdateFileFile.LastWriteTime)).TotalHours < 24)
{
// do your logic here...
}
// you could also get at DateTime.Now.Date() or Day.. depending on what you want to do

Parsing times above 24 hours in C#

Suppose a time stamp (just time or date and time) where the time can roll over to the next day:
00:00:00 <- midnight
01:00:00 <- 1 AM
23:00:00 <- 11 PM
24:00:00 <- midnight, day + 1
25:00:00 <- 1 AM, day + 1
What would be a way to parse it easily into a C# DateTime that would perform the carry-over to the next day? In other words, "01:00:00" would become "0001-01-01 01:00:00" and "25:00:00" would become "0001-01-02 01:00:00".
EDIT:
I should mention that this fails miserably (i.e FormatException):
DateTime.ParseExact("0001-01-01 25:00:00", "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);

Since you're trying to represent a period of time from an arbitrary point, rather than as a specific date, perhaps you would be better off using the System.TimeSpan class? This allows you to set values of more than 24 hours in the constructor, and can be used with DateTime objects like this:
System.TimeSpan timestamp = new System.TimeSpan(25, 0, 0);
System.DateTime parsedDateTime = new DateTime(0, 0, 0);
parsedDateTime = parsedDateTime.Add(timestamp);
Console.WriteLine(parsedDateTime.ToString("yyyy-MM-dd HH:mm:ss")); //Output as "0001-01-02 01:00:00"
NOTE: Code is untested.
EDIT: In terms of parsing the strings, I can't think of any basic .NET objects that parse strings with values greater than 23 for the hour (since 25 is an invalid hour of the day), but assuming that the format is consistent, you could create a very simple string parsing routine (or even a regular expression) to read the values individually, and load the constructor manually.

If you have an existing DateTime value you can add to, you can always use a TimeSpan:
string dt = "25:00:00";
int hours = int.Parse(dt.Split(':')[0]);
TimeSpan ts = TimeSpan.FromHours(hours);
TimeSpan.Parse() doesn't work directly in this case because it complains (fair enough!) about the 25 in the hour notation.

If you want to code it out... this should be a starting point:
string dateString = "0001-01-01 25:00:00";
string[] parts = dateString.Split(' '); //now have '0001-01-01' and '25:00:00'
string datePart = parts[0]; // '0001-01-01'
string[] timeParts = parts[1].Split(':'); //now have '25', '00', and '00
DateTime initialDate = DateTime.ParseExact(datePart, "yyyy-MM-dd", CultureInfo.InvariantCulture);//use the date as a starting point
//use the add methods to get your desired datetime
int hours = int.Parse(timeParts[0]);
int minutes = int.Parse(timeParts[1]);
int seconds = int.Parse(timeParts[2]);
DateTime resultDate = initialDate.AddHours(hours)
.AddMinutes(minutes)
.AddSeconds(seconds);
Of course, it makes assumptions that the input is formatted properly and is parsable, etc..
In addition, you could definitely use timespan instead of the individual add methods for hour, minute, second as some other answers are..

In case nobody points out an out-of-the-box answer, here is a neat ActionScript class I wrote to parse time inputs (human input)...
https://github.com/appcove/AppStruct/blob/master/Flex/AppStruct/src/AppStruct/TimeInput.as
It would be very simple to port this to C#, and you could tweak the 24 hour logic to result in #days, #hours, #minutes.
Good luck!

You are specifying an invalid date. So not only can you not parse it, you cannot store it!
How about a nice TimeSpan object instead? (It also has a Parse() method.)
Alternatively, use a sscanf()-type function like the one at http://www.blackbeltcoder.com/Articles/strings/a-sscanf-replacement-for-net to extract each number separate. (Best if you have no control over the string format being read.)