On Debian 13 (systemd-based), a resume from hibernation is recorded as a new boot in the journal, so the most recent resume time is simply the start time of the current boot — provided the previous boot actually ended with hibernation. Here's how to check:

1. Get the most recent resume/boot time

journalctl --list-boots

The top (or bottom, depending on version) entry is the current boot; its start timestamp is when the system resumed from hibernation. Or just:

last -x reboot | head -1

2. Confirm the previous boot ended in hibernation

Check the end of the previous boot:

journalctl -b -1 -u systemd-hibernate.service

You should see a "System hibernation" entry. (If it was suspend-to-RAM instead, the unit would be systemd-suspend.service — but suspend doesn't create a new boot entry, so a new boot after a short gap usually means hibernation.)

3. Verify from the kernel log of the current boot

journalctl -b 0 -k --grep "PM: hibernation exit"

The kernel prints PM: Hibernation exit right after resuming from disk, and the timestamp of that line (or the first line of journalctl -b 0) is your resume time. If nothing was hibernated, this line will be absent.

So in short: journalctl --list-boots (or last -x reboot) for the time, plus the systemd-hibernate.service / PM: hibernation exit checks to confirm it was really a hibernation resume rather than a cold boot.