Generally, it uses clock.now or a calculation based on clock.now, but it can take any value that can be interpreted as a date including dates in string form.
Action Sets the modification date of the file.
Any operation that changes a file automatically updates its modification date. You can get the modification date using the file.modified verb. A file's modification date is very important - it can be used for backup scripts. Generally speaking, you should not explicitly change a file's modification date.