Do not invent an offset
Two timestamps can only support a precise comparison if the file gives enough reason to treat their time bases as comparable. This does not mean assuming every clock is wrong. It means avoiding a made-up adjustment whenever a preferred theory fails to fit. Keep the displayed times intact and note any explicit evidence about clock settings, corrections or synchronization in the fictional materials.
Worked example: eight minutes ahead
A workshop receipt at 17:08 conflicts with a wall-clock photograph apparently showing 17:00. A maintenance note might establish that the wall clock was eight minutes slow. Without that note or another supported reference, changing one time by eight minutes merely makes the records fit your theory. It does not explain them. The apparent discrepancy should remain a question until the evidence narrows it.
Test the adjustment elsewhere
If the file supports a clock offset, apply it consistently to every relevant event from that clock, not only to the entry causing trouble. Check whether the adjusted sequence fits other independent anchors. Record both the displayed time and the interpreted time so you can reverse the calculation if necessary. In a cold case puzzle, a clock explanation is strongest when it accounts for several observations without requiring a different convenient correction for each one.