What does the 'Z' mean in Unix timestamp '120314170138Z'?

Question

I have an X.509 certificate which has the following 2 timestamps:

['validFrom'] = String(13) "120314165227Z"
['validTo']   = String(13) "130314165227Z"

What does the postfix character 'Z' mean. Does it specify the timezone?

It normally indicates "zulu" time, aka, UTC/GMT. see: http://meta.stackexchange.com/questions/15986/tool-tip-timestamp-shows-a-z — Marc B, Mar 14 '12 at 17:13
What produced that output, and why is it using a 2-digit year? — Keith Thompson, Mar 14 '12 at 18:19
@Keith Thompson It was produced in PHP 5.2 with: var_dump(openssl_x509_parse($certString)) — HomeCoder, Mar 14 '12 at 21:49

Sogger · Accepted Answer · 2012-03-14T17:42:51.253

228

Yes. 'Z' stands for Zulu time, which is also GMT and UTC.

From http://en.wikipedia.org/wiki/Coordinated_Universal_Time:

The UTC time zone is sometimes denoted by the letter Z—a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see time zone history). Since the NATO phonetic alphabet and amateur radio word for Z is "Zulu", UTC is sometimes known as Zulu time.

Technically, because the definition of nautical time zones is based on longitudinal position, the Z time is not exactly identical to the actual GMT time 'zone'. However, since it is primarily used as a reference time, it doesn't matter what area of Earth it applies to as long as everyone uses the same reference.

From wikipedia again, http://en.wikipedia.org/wiki/Nautical_time:

Around 1950, a letter suffix was added to the zone description, assigning Z to the zero zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which pronounces the letter Z as Zulu, leading sometimes to the use of the term "Zulu Time". The Greenwich time zone runs from 7.5°W to 7.5°E longitude, while zone A runs from 7.5°E to 22.5°E longitude, etc.

edited Mar 14 '12 at 17:42

answered Mar 14 '12 at 17:15

Sogger

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what is the abbreviation for PDT and PST – viper Jul 07 '16 at 10:51
It totally depends on your context and what format your destination is expecting. If you are trying to map the PDT and PST to Nautical time zone values, use T and U respectively (http://www.timeanddate.com/time/zones/) However, remember that even though the offset values are the same the geographical areas they represent are not. – Sogger Jul 11 '16 at 18:56
4

This answer contradicts the references. Assuming the Wikipedia pages are accurate, "z" doesn't stand for "zulu", it stands for "zero" since UTC is the "zero" timezone. "zulu" is just the NATO phonetic alphabet word for "z". In other words we call it "zulu time" because of the "z" as opposed to the inverse this answer suggests which is that the "z" is used because it's a shorthand for "zulu time". Again though, assuming those references are accurate. I don't have any additional outside knowledge on the subject – mowwwalker Feb 22 '21 at 22:53
flawless answer! – Gaurav Apr 27 '21 at 11:23
While accurate for GMT, it is a common miss conception that it is the same for UTC. Also UTC is not a time zone as this answer seems to make it sound out to be. It is however common to say UTC, when referring to UTC with Zero zone, but as it is stated in this answer it may misguide the reader. – Tolga Jun 16 '21 at 02:20

score 34 · Answer 2 · answered Feb 25 '21 at 17:57

"Z" doesn't stand for "Zulu"

I don't have any more information than the Wikipedia article cited by the two existing answers, but I believe the interpretation that "Z" stands for "Zulu" is incorrect. UTC time is referred to as "Zulu time" because of the use of Z to identify it, not the other way around. The "Z" seems to have been used to mark the time zone as the "zero zone", in which case "Z" unsurprisingly stands for "zero" (assuming the following information from Wikipedia is accurate):

Around 1950, a letter suffix was added to the zone description, assigning Z to the zero zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications — zones M and Y have the same clock time but differ by 24 hours: a full day). These can be vocalized using the NATO phonetic alphabet which pronounces the letter Z as Zulu, leading to the use of the term "Zulu Time" for Greenwich Mean Time, or UT1 from January 1, 1972 onward.

Best answer. Wish more people would take to time to fact check their knowledge instead of assuming something is true just because they hear everyone else saying it. — Tolga, Jun 16 '21 at 02:12

score 25 · Answer 3 · answered Mar 14 '12 at 17:13

The Z stands for 'Zulu' - your times are in UTC. From Wikipedia:

The UTC time zone is sometimes denoted by the letter Z—a reference to the equivalent nautical time zone (GMT), which has been denoted by a Z since about 1950. The letter also refers to the "zone description" of zero hours, which has been used since 1920 (see time zone history). Since the NATO phonetic alphabet and amateur radio word for Z is "Zulu", UTC is sometimes known as Zulu time. This is especially true in aviation, where Zulu is the universal standard.

What does the 'Z' mean in Unix timestamp '120314170138Z'?

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