Monday, January 31, 2005

DateDiff in C#

This post will probably only matter to those of us who have VB roots, which I get no end of heckling from my co-workers about. But as I was working today, one of the things I had to do was calculate the span between two dates in months. Piece of cake, I thought to myself. All I have to do is use the DateDiff and specify months...except where is DateDiff in C#??? It is AWOL!

So I tried to see what else I could come up with. I did uncover this thing called System.Timespan. Apparently, Timespan will give you the difference between two DateTimes in something called 'ticks.' You can then take the ticks, and convert them to days, hours, minutes, seconds, and milliseconds. The drawback is that if you want the difference in years or months, you have to do some hoaky manual calculation that may or may not jibe with what the span really should come out to. In my research, this was the best example of how to duplicate the DateDiff in C#.


Anonymous said...

Um, why not use DateDiff itself?

Add a reference to Microsoft.VisualBasic, add:

using Microsoft.VisualBasic;

Then in a form (three textboxes & a button)

private void button1_Click(object sender, System.EventArgs e)
DateTime FromDate;
DateTime ToDate;
FromDate = DateTime.Parse(this.textBox1.Text);
ToDate = DateTime.Parse(this.textBox2.Text);

this.textBox3.Text = DateAndTime.DateDiff (DateInterval.Month, FromDate,ToDate, FirstDayOfWeek.System, FirstWeekOfYear.System ).ToString();

The only bummer is C#'s lack of support for Optional parameters, so you have to code FirstDayOfWeek.System & FirstWeekOfYear.System as the last two parameters. (A little confusing if you're used to VB :-)

Valerie Vogt said...

You know, I honestly had not even considered adding a reference to VB. I do like the idea of not having to assume that the number of days divided by 365 and multiplied by 12 will give me the correct number of months.

I wonder if there would be any negative side effects of this option when Whidbey is released...I suppose if I wanted to port anything over to .Net 2.0, I will most likely have to make some code fixes to upgrade my C# code to the 2.0 wouldn't be a big deal to have to do the same with the VB code.

Thanks for the idea :)

Justin J. Vogt said...

public static int CalculateMonthDifference(DateTime startDate, DateTime endDate)
int monthsApart = 12 * (startDate.Year - endDate.Year) + startDate.Month - endDate.Month;
return Math.Abs(monthsApart);


Anonymous said...

It comes from writing projects in both VB & C#. As much as there is a potential of a semi-religious war over these two languages (let alone adding Java, perl, python etc into the mix) I end up using both at various times.

I have many VS solutions w/ mixed VB & C# projects, depending on what I'm doing.

The DateDiff & other VB "holdovers" are a great examples, but my favorite is the optional parameter issue. I don't even think about doing office automation in C# (see;EN-US;316384 vs;EN-US;316383 and the need of "ref oMissing" in C#) because of this.

On the other hand - there's something that I just like about the look of C#'s code :-)

Christopher G. Lewis

smith said...

I see folks saying Timespan is the C# Answer to DateDiff in so many places. It's NOT the same result and the difference is pretty important.

If you ask DateDiff the number of hours between today at 1pm and today at 2pm DateDiff will return 1. TimeSpan will do the same.

Now ask DateDiff the number of Minutes between 13:00:00 and 14:00:00 and it will return 60. Timespan will return Zero because there is ONE Hour difference and ZERO Minutes difference. Each section of Timespan is discrete, not so with DateDiff; DateDiff calculates the total at the level you ask for.

//here's hours:

DateTime dtStart = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day,13,0,0,0);

DateTime dtEnd = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day,14,0,0,0);

TimeSpan elapsed = dtEnd.Subtract(dtStart);

// no matter how you call it
TimeSpan ts = dtEnd - dtStart;
MessageBox.Show( "diff = " + ts.Hours.ToString() + " hours" );

// now try with with minutes:

DateTime dtStart = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day,13,0,0,0);

DateTime dtEnd = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day,14,0,0,0);

TimeSpan elapsed = dtEnd.Subtract(dtStart);

// no matter how you call it
TimeSpan ts = dtEnd - dtStart;
MessageBox.Show( "diff = " + ts.Minutes.ToString() + " minutes" );

Here's a C# DateDiff example.

All those lines and all of those Casts, yeah, that's so much more "Pretty" than including a reference to a core DLL (visualbasic.dll is core, just like System. You can't install the Frameowrk without it)

I do understand that some folks don't want to include any extra dlls because of memory overhead one they're referenced in the primary AppDomain, and that's fine (yet still could be gotten around with less code by creating a new AppDomain, loading the VB dll into it, calling the function then dropping the AppDomain and never knowing that you had loaded the dll after the fact. It's not too hard and only about the same amount of code as the C#-only way).

But if a person is only avoiding the VisualBasic.Dll because of its name then they're just plain adding work where it's not needed, and I'd think twice about putting them on a hire list because I look for ".NET developers" who 'get the Whole framework', not "VB" or "C#" developers.

All the best.

smith said...

(VAL the EDITOR Gal: you can attach this to my prevous post if you like]

btw: before it's mentioned, my example was in a time part so using ts.TotalSeconds, ts.TotalMilliseconds, etc on time parts can get you those calculated sums, but DateDiff isn't limited to time intervals and so along with TimeSpan there should be a Framework DateSpan to fill in the gaps ... to my knowledge that's not there so DateDiff is still a functional winner.

Anonymous said...

Thanks, very good idea.

Yodan Tauber said...

The problem with calclulating date difference in months is that a "month" is an abstract definition for a period of time. Unlike "second", which is a very well defined period of time, and unlike "hour", which is exactly 3600 seconds, "month" could either mean 28, 29, 30, or 31 days (and that's just in the Gregorian calendar! what about the Hebrew or Hijri calendars?).

How would you define "difference in months"? Is it just a difference of roughly 30 days, or is it any difference in the current month of the calendar? What is the difference in months between April 30th and May 1st?
May 1st and May 31st? Is the difference between February 1st and March 1st the same as the difference between July 1st and August 1st?

Each of the suggested ways to calculate the difference in months between two days would be imperfect, becase the difference in months just can't be calculated in a solid formula. You should choose your method by your own definition of "difference in months".

Pat said...

I used to be a VB coder too but please do not, ever use that DateDiff from Microsoft.VisualBasic.

If I give it a date of 21/03/2006 and a date of 21/01/2007 and ask for the diff in years it says 1!

It is not a year it is only 10 months. I checked the code out in reflector and all it is doing is getting the 2 year parts and subtracting them. OMIGOD that is poor.

Pat said...

Just found this on the MSDN forums

Check the code from Ti starting
"sDay = oDateTime.Day"

Monty said...

Actually it's quite easy...

int numMonths = 12 * (endDate.Year - startDate.Year) + endDate.Month - startDate.Month + 1;

Anonymous said...


you asked for the diff in years...
why would you expect to get a diff of 10 months?

the diff in years between dec 31 2007 and jan 1 2008 is 1.

if you wanted to check for a span of 12 months, you should have asked for a diff in months. if you wanted to check for a span of 365 days, you should have asked for days.

don't fault the language because of your own misunderstandings.

Valerie Vogt said...

I didn't even provide a code sample, so I am confused by your remark. My point was that there wasn't a comparable C# DateDiff to the VB.Net DateDiff.

Stephen said...

here's a short function I made to calculate month difference - this one rounds up to the next month - remove the +1 if you want it to count 09/03/2010 - 09/04/2010 as 1 month - at present it counts 09/03/2010 - 08/04/2010 as 1 month - if that makes sense? ps I use uk date format :)

public int GetMonthSpan(DateTime startDate, DateTime endDate)
int years = endDate.Year - startDate.Year;
int months = endDate.Month - startDate.Month + (12 * years);
int days = endDate.Day - startDate.Day + 1;
return (days > 0) ? months + 1 : months;