CST is 6 hours behind Zulu, and CDT is 5 hours behind Zulu. Live clocks, the full hour chart and the daylight saving rule that catches people out.
Zulu time is UTC — the clock every flight plan, order and server log is written in, precisely because it never moves. Central Standard Time (CST) sits UTC−6, which makes it 6 hours behind Zulu. Converting one to the other is a single addition, and the only two things that go wrong are the date and daylight saving.
The converter above tracks the current time until you type into it. Enter any Zulu time — 1730Z, 17:30Z or NOW — and it renders the same instant in CST in 24-hour, 12-hour and dated form. The chart below is the same conversion for every hour of the day, so it can be read without typing anything at all.
Every hour from 0000Z to 2300Z. Two columns, because the zone has two offsets: read the CST column in winter and the CDT column from the second Sunday in March to the first Sunday in November. A single column here would be wrong for most of the year. Where the sum crosses midnight the date moves with it, marked in the cell.
| Zulu | CSTUTC−6 | CDTUTC−5 |
|---|---|---|
| 0000Z | 18:00 −1 day | 19:00 −1 day |
| 0100Z | 19:00 −1 day | 20:00 −1 day |
| 0200Z | 20:00 −1 day | 21:00 −1 day |
| 0300Z | 21:00 −1 day | 22:00 −1 day |
| 0400Z | 22:00 −1 day | 23:00 −1 day |
| 0500Z | 23:00 −1 day | 00:00 |
| 0600Z | 00:00 | 01:00 |
| 0700Z | 01:00 | 02:00 |
| 0800Z | 02:00 | 03:00 |
| 0900Z | 03:00 | 04:00 |
| 1000Z | 04:00 | 05:00 |
| 1100Z | 05:00 | 06:00 |
| 1200Z | 06:00 | 07:00 |
| 1300Z | 07:00 | 08:00 |
| 1400Z | 08:00 | 09:00 |
| 1500Z | 09:00 | 10:00 |
| 1600Z | 10:00 | 11:00 |
| 1700Z | 11:00 | 12:00 |
| 1800Z | 12:00 | 13:00 |
| 1900Z | 13:00 | 14:00 |
| 2000Z | 14:00 | 15:00 |
| 2100Z | 15:00 | 16:00 |
| 2200Z | 16:00 | 17:00 |
| 2300Z | 17:00 | 18:00 |
Take the four digits, subtract 6 hours, and roll the date if the result falls outside 00:00–23:59. 1730Z becomes 11:30 CST; 0200Z becomes 20:00 CST the previous day.
Between the second Sunday in March to the first Sunday in November the zone is on Central Daylight Time, so the correction becomes subtract 5 hours instead. Same instant, different local label — 1730Z is 12:30 CDT.
Do the arithmetic in Zulu and convert once, at the end, for whoever needs a local time. Adding en-route time to a departure slot in local time and converting afterwards is how the day boundary gets lost. The Zulu time converter does both halves in one box.
CST is also an abbreviation for China Standard Time (UTC+8) and for Cuba Standard Time. This page means the North American Central zone. If the message came from a Pacific-theatre unit, check which CST is meant before converting.
Zulu itself never shifts, so every hour of the shift lands on the local side of the conversion: UTC−6 in winter, UTC−5 from the second Sunday in March to the first Sunday in November. If a message does not say which, use the date it was written on to decide, and read the matching column above.
Chicago, Dallas, Houston, New Orleans and a large slice of the US interior, including Fort Cavazos and the Gulf Coast air stations. Also Winnipeg and central Mexico.
Flight plans, METARs, TAFs, NOTAMs and ATC clearances are all in Zulu, and so are military orders, satellite passes and incident timelines. The conversion to CST is the last step, for the people who have to be somewhere at a local hour — a briefing time, a crew report, a maintenance window. Written the aviation way it carries the day of the month too: 141730Z is the 17:30 Zulu of the 14th, which removes the last ambiguity a date shift can create. See military time for the four-digit notation itself.
CST is 6 hours behind Zulu (UTC−6). To convert, take the Zulu time and subtract 6 hours: 1200Z is 06:00 CST. During Central Daylight Time it is 07:00 CDT instead.
The abbreviation CST always means UTC−6, but the clock it describes switches to CDT (UTC−5) from the second Sunday in March to the first Sunday in November. Zulu never changes, so the gap between them is an hour smaller during CDT.
0000Z is 18:00 CST the previous day. The date rolls back, which is the shift people most often lose. On daylight time it is 19:00 CDT the previous day.
Yes, for every practical purpose. Zulu is how UTC is written and spoken in aviation and military use — the Z suffix is the phonetic letter for the zero-offset zone. GMT is the older name for the same clock reading, though the UK itself moves to BST in summer.
Reverse the arithmetic: add 6 hours and add the Z suffix. Do the maths in Zulu first and convert once at the end — converting first and adding afterwards is where the date boundary gets lost.