Actually, it's the last modified for a file, from the moment the script started. If your script has been running for 48 hours (it could happen), then -M is totally off course. stat is in this case a much safer approach.
Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.
Comment on Re: Answer: How do I interpret mtime with stat()?
Well, actually the filetest operators compute their deltas against the value of $^T, which is initialized during startup. However, you may reset it if you think it's value might be getting stale: