Problems with date() - what have I missed?

Hi list. I have a strange problem with a PHP script (WinNT/Apache/PHP4/MySQL). The line of code $dayname = date("D", mktime(0,0,0,$cmonth,$eventdate,$cyear)); should, if I understand it correctly, fill the variable $dayname with the three-letter abbreviation of the day of the week for the date I supply. I have $cmonth=3, $eventdate=26, and $cyear=2001. I would expect this to put "Mon" in $dayname, given that March 26th 2001 is a Monday. However, my script displays "Tue" which is a bit peculiar. As if that weren't enough, it isn't consistent in getting the days wrong. For example: November 30th 2000 was a Thursday. It gets this right. December 14th 2000 was a Thursday. It says it was a Tuesday. January 11th 2001 was a Thursday. It says it was a Monday. February 11th 2001 was a Sunday. It says it was a Monday. March 26th 2001 is a Monday. It says it is a Tuesday. What am I doing wrong? Is it something to do with last year (officially it wasn't a leap year but as 2000 is divisible by 4, many people thought it should be)? I'm in GMT so unless it's a BST thing I'm not being affected by a local time zone. All help gratefully appreciated.... Cheers Jon