Sync w/ time span not reliable

[chota300]

I have a folder where I mirror only files created/updated in the last "n" days.
This does not work reliably.
I delete all the files in the target folder ( by copy of empty folder ), then in a subsequent step copy with a days limit.
Results are inconsistent. I get a different subset of the selected files in repeated executions.
Something is broken.
Since I first empty the target folder, the copy of files from the last "n" days should be reliable and consistent.