I usually use the simpler

if ((-M '/tmp/file') > $ageThreshold) {
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to test the modify time. (or -A or -C for the access or change time on unix). See the documentation for file tests.

Two things to note

• The time is given in days, not hours or minutes.
• The time is from the start of the script. That cuts overhead if you're checking more than one file, but it's a problem if you're waiting for the file to be old enough

Ususally, neither of these is a problem.