filemtime doesn't always copy correctly

I am backing up a file (the path & filename of which are stored in $original; the path & filename of the copy are stored in $bkup). I would also like to copy the file modification time of $original, so, after copying the file itself, I use: touch($bkup, filemtime($original)); However, after I use the above "touch" function, filemtime($original) and filemtime($bkup) are often slightly different. For example, I might have: echo filemtime($original); // gives 1126191599 echo filemtime($bkup); // gives 1126191600 Why the small difference? (I'd like these two file modification times to be exactly the same because I want to compare file modification times when I do incremental backups, and copy a file only if the file modification times are different.) By the way, if it helps, I am using Windows XP. Thanks!