Zone letters: how the patrol reports kept time

Every time in a patrol report ends in a letter: 2256 K, 0330 Z, 1600 VW. The letter is the time zone the clock was set to. Read without it, two times in the same report can be hours apart for the same moment.

The letters

The Navy split the world into 24 zones, each 15° of longitude wide and one hour apart, centred on Greenwich (0°), 15°, 30° and so on. Each zone has a letter. Z is Greenwich. Going east, where clocks are ahead of Greenwich, the letters run A to M (there is no J): K is 10 hours ahead, I is 9. Going west, where clocks are behind, they run N to Y: W is 10 hours behind, Y is 12. M and Y share the 180th meridian. They show the same clock time, a day apart.

The reports also give zones as numbers, with the sign the other way round. A zone's number, its zone description, is what you add to local time to get Greenwich time. So K, ten hours ahead, is zone −10: 2256 K minus 10 hours is 1256 Z. Greenwich time was called GCT, Greenwich Civil Time (today's UTC).

A clock is not where the boat is

The bands on the map are where each zone's clock agrees with the sun. A ship's clock didn't have to. The captain chose the zone, and changed it when it suited him, usually on the way out and back. Click K above. On her first patrol BANG kept K, the zone of Guam and Saipan, all the way to the Luzon Strait, two bands to the west, where a K clock read about 2 p.m. at local noon. She kept K again on Patrols 3 and 4. On Patrol 2 she changed to I on 22 June 1944 as she went west, and on Patrols 5 and 6 she kept I on station. The three boats of her first wolf pack, in the same waters in April and May 1944, kept three different zones: BANG K, PARCHE I, TINOSA H (choose "Other submarines").

So the letter has to be read off each page, even each table. The narrative is in the zone the boat was keeping. Tables are often in Z, Greenwich time. BANG's bathythermograph (density-layer) tables are, so her Greenwich-time positions are spread over every band (click Z). Japanese records are all in Tokyo time (I), which the Imperial Navy kept across its whole area. So do the FRUPAC radio-intelligence notes, as far as the matches show.

One moment, six clocks

Pick a moment from the reports to see what every clock in the story read at that instant.

The worked example

BANG's first attack, on convoy TAMA-17 in the Luzon Strait: her attack sheet times the first torpedo at 2256 K on 29 April 1944. The Japanese report of Takegawa Maru's sinking puts the hit at 2201 on the 29th, in Tokyo time (I). Taken at face value the two are 55 minutes apart. Converted, they are 5 minutes apart:

Record	As written	Zone description	Greenwich (GCT)
BANG, attack sheet	29 Apr 2256 K	−10	29 Apr 1256 Z
Japanese report, Takegawa Maru	29 Apr 2201 I	−9	29 Apr 1301 Z

Every time on the map, dive-profile and battle pages is converted this way (tools/check_track.py, its ZONES table) and shown with its letter as typed. The raw time is never changed in the CSVs.

Odd letters you'll meet

• VW, Hawaii. A half-hour zone, written with the two letters it falls between. Hawaii kept GMT−10½ before the war. From February 1942 it kept "war time", an hour ahead: GMT−9½, zone +9½. Every departure from Pearl Harbor is in VW.
• KL and IK: half-hour zones again, GMT+10½ and GMT+9½, used by GROWLER on her Patrol 9. USS Haynsworth's April 1945 war diary steps through WX and UV on its way to Hawaii.
• Y and M: the same clock time on opposite sides of the date line. Midway's departures are in Y. Crossing westward, the date jumps a day ahead. Haynsworth, sailing east, wrote 23 April 1945 twice: once in M, once in Y.
• Z in the middle of the ocean: a table kept in Greenwich time, not a position near Greenwich.

The same letters reach Europe. The Army records date their entries in A (GMT+1) and, in summer, B (GMT+2). See the Europe page on map grids.

Counts

Positions in the transcribed CSVs, by the zone letter typed with them.