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C#: Code Optimization for Date Formatting

Using C#, I am trying to populate a variable with a known filename + date. The catch is that the date must always be the last day of the previous month. It must also have no slashes or hyphens, and the year must be two digits. For example, if it is now November 27 2015, I need my filename to be: Foobar_103115.txt
As a programmer who still has much to learn, I have written the clunky code below and it does achieve my desired result, even though it will obviously break after the end of this century. My code is written this way because I could not figure out a more direct syntax for getting the date I want, complete with the specified formatting.
My question is this: What would be a more elegant and efficient way of recreating the below code?
I have commented all the code for any novice programmers who might be interested in this. I know the experts I'm asking for help from don't need it.
public void Main()
{
String Filename
DateTime date = DateTime.Today;
var FirstDayOfThisMonth = DateTime.Today.AddDays(-(DateTime.Today.Day - 1)); //Gets the FIRST DAY of each month
var LastDayOfLastMonth = FirstDayOfThisMonth.AddDays(-1); //Subtracts one day from the first day of each month to give you the last day of the previous month
String outputDate = LastDayOfLastMonth.ToShortDateString(); //Reformats a long date string to a shorter one like 01/01/2015
var NewDate = outputDate.Replace("20", ""); //Gives me a two-digit year which will work until the end of the century
var NewDate2 = NewDate.Replace("/", ""); //Replaces the slashes with nothing so the format looks this way: 103115 (instead of 10/31/15)
Filename = "Foobar_" + NewDate2 + ".txt"; //concatenates my newly formatted date to the filename and assigns to the variable

Sounds like you want something more like:
// Warning: you should think about time zones...
DateTime today = DateTime.Today;
DateTime startOfMonth = new DateTime(today.Year, today.Month, 1);
DateTime endOfPreviousMonth = startOfMonth.AddDays(-1);
string filename = string.Format(CultureInfo.InvariantCulture,
"FooBar_{0:MMddyy}.txt", endOfPreviousMonth);
I definitely wouldn't use ToShortDateString here - you want a very specific format, so express it specifically. The results of ToShortDateString will vary based on the current thread's culture.
Also note how my code only evaluates DateTime.Today once - this is a good habit to get into, as otherwise if the clock "ticks" into the next day between two evaluations of DateTime.Today, your original code could give some pretty odd results.

Count forward and check date forward

this is how I would like that to build up such that it has the date today d 2 / 23-2015 and so it goes 30 days ahead of the date, which means it will be d 22.02.2015,
I've tried to do like this:
string datoTid = DateTime.Now.ToString("dd-MM-yyyy");
DateTime equalsDato = datoTid.AddDays(1 * 30);
string slutdato = equalsDato.ToString("dd-MM-yyyy");

This should work:
string slutdato = DateTime.Now.AddDays(1 * 30).ToString("dd-MM-yyyy");

I'm not entirely sure what your asking, I believe your intent is to take the current date and added thirty days. Which would be:
var date = DateTime.Now.AddDays(30);
This would provide a type of DateTime. Then you can create a string representation of the designated DateTime. (In your desired format)
date.ToString("dd-MM-yyyy");
This would provide your modification for the format, while simply applying the designated date ahead.

Getting only hour/minute of datetime

Using C#, I have a datetime object, but all I want is the hour and the minutes from it in a datetime object.
So for example:
if I have a DateTime object of July 1 2012 12:01:02 All I want is July 1 2012 12:01:00 in the datetime object (so, no seconds).

Try this:
String hourMinute = DateTime.Now.ToString("HH:mm");
Now you will get the time in hour:minute format.

Try this:
var src = DateTime.Now;
var hm = new DateTime(src.Year, src.Month, src.Day, src.Hour, src.Minute, 0);

Just use Hour and Minute properties
var date = DateTime.Now;
date.Hour;
date.Minute;
Or you can easily zero the seconds using
var zeroSecondDate = date.AddSeconds(-date.Second);

I would recommend keeping the object you have, and just utilizing the properties that you want, rather than removing the resolution you already have.
If you want to print it in a certain format you may want to look at this...That way you can preserve your resolution further down the line.
That being said you can create a new DateTime object using only the properties you want as #romkyns has in his answer.

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.)