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Watchkeeping

Ship’s clocks and time zones

How far to put the ship's clocks forward or back when changing zone, what happens to the date at the date line, and how to share the change fairly between the watches.

UTC offset (east positive) or a longitude with E/W

Steaming
watches

1 to change at once

Fill in the fields to see the result.

How it works

Zones are 15° of longitude wide, centred on multiples of 15°: zone = longitude / 15, rounded. Steaming east, clocks are advanced; west, they are retarded. The zone description (ZD) is the correction from ship's time to UTC: UTC+3 is ZD −3.

Zones +12 and −12 meet at the 180th meridian (the date line, which bends around some islands). Crossing it eastward the date repeats (a day is lived twice); westward a day is skipped. The ship's clocks follow the zone; the company or master decides when to change them and how to share the change (often 20 minutes in each of the three night watches).

For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.

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