Date addition problem

Hi I am having an unusual problem when trying to calculate dates in advance from a start date. the code below shows a loop where by on each run an increasing number of weeks is added to the start date, this works as expected up untill the 8th time where for some reason it produces 01-11-05 instead of 02-11-05. I am at a loss as to why this would happen when it works perfectly for all the other dates. where am i going wrong? [snip] <?php echo"<h1> date test</h1>"; $start = "05-09-07"; $start = explode('-',$start); $startmk = mktime(0,0,0,$start[1],$start[2],$start[0]); $startdate = date('d-m-y',$startmk); for($i=0;$i<10;$i++){ $nextdate = date('d-m-y',$startmk + ($i*604800)); echo"<h1>$i: $startdate -- -- --$nextdate</h1>"; } ?> OUTPUT: 0: 07-09-05 -- -- --07-09-05 1: 07-09-05 -- -- --14-09-05 2: 07-09-05 -- -- --21-09-05 3: 07-09-05 -- -- --28-09-05 4: 07-09-05 -- -- --05-10-05 5: 07-09-05 -- -- --12-10-05 6: 07-09-05 -- -- --19-10-05 7: 07-09-05 -- -- --26-10-05 8: 07-09-05 -- -- --01-11-05 9: 07-09-05 -- -- --08-11-05 [/snip] Thanks a lot Ade