If you need to figure out the start of a day based on a filedate, You need to do some math
Or you can make the program do it for you
@var $filedate = GetFileTime( $PathToFile );
// Get date as string
@var $date = FormatDate( "yyyy-MM-dd" , $filedate );
// parse date to num value.
@var $startOfDay = ParseDateTime($date, "%Y-%m-%d");