A photograph says 16:20, your notebook says 15:20, and both seem to describe the same harbour visit. Before deciding that one record is wrong, ask what each clock was showing. A bare time can lose its meaning when a trip crosses a time zone or a daylight-saving change.
For a personal sailing journal, you do not need an elaborate database. You need a short convention that your future self can read without reconstructing the settings on an old phone.
Keep the clock reading and its context together
Write the date, time and offset on the same line: “5 October, 16:20, UTC+02:00.” In this illustrative example, subtracting two hours gives 14:20 UTC on the same date. The offset belongs to that observation; do not apply it automatically to another month or another place.
NIST explains the relationship between UTC and local time, including the fact that UTC itself does not move for daylight saving. That distinction is useful when two records use different clock conventions.
Check the date when the calculation crosses midnight
If a note reads 00:30 on 6 October at UTC+02:00, its UTC equivalent is 22:30 on 5 October. Write both dates when converting. An hour correction that silently leaves the date unchanged creates a new error while appearing to tidy the old one.
Keep the original entry alongside any conversion. A second column headed “converted to UTC” lets you review the arithmetic without losing what was actually written at the time.
Treat a photo timestamp as something to check
A camera may have kept an earlier setting. Compare its displayed time with a reliable reference before using it to order events. If you cannot establish the offset afterwards, mark the timestamp as uncertain rather than choosing the most convenient answer.
Add a place name and a brief description of the event too. Time alone will not tell you whether a picture shows arrival, departure or a later walk along the quay. Our guide to writing a useful sailing-photo caption offers a simple companion routine.
For navigation, forecasts and official records, follow their stated time convention and applicable procedures. This notebook habit is for keeping your own observations understandable, not for replacing those instructions.