YYYY-MM-DDTHH:MM:SS.SSSZ is a date-and-time timestamp: the date comes first, T separates it from the clock time, .SSS gives milliseconds, and Z is the UTC designator. The pattern describes a millisecond-precision form of the RFC 3339 date-time format; RFC 3339 itself permits other numbers of fractional-second digits.
How to read each part of the timestamp
For example, 2025-03-08T14:05:09.327Z represents the date and time 8 March 2025 at 14:05:09.327, marked with Z. Read the fields from left to right:
| Part | Meaning |
|---|---|
YYYY |
Four-digit year. |
MM after the year |
Two-digit month, from 01 through 12. |
DD |
Two-digit day, valid for that month and year. |
T |
Separator between the date and time. |
HH |
Hour, from 00 through 23. |
MM after the hour |
Minute, from 00 through 59. |
SS |
Second, normally from 00 through 59; 60 is permitted for a leap second under the specification’s rules. |
.SSS |
Three fractional-second digits: thousandths of a second, commonly called millisecond precision. |
Z |
UTC designator, with an important standards nuance described below. |
RFC 3339 is an Internet date-time profile based on ISO 8601. Its syntax allows one or more fractional-second digits when a fraction is present, so three digits are a common precision choice rather than a requirement of the standard. See the RFC 3339 specification.
What Z means—and the standards update
In everyday use, Z is commonly understood as indicating UTC. RFC 3339 originally defined it as a zero UTC offset and distinguished it from -00:00, which indicated that UTC time was known but the local offset was unknown. RFC 9557 later updated RFC 3339: Z can express that UTC time is known while the local offset is unknown, while +00:00 continues to mean that UTC is the preferred reference point. The intended meaning can therefore depend on the standard and application that produced the timestamp. See RFC 9557.
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How to validate the timestamp
A string can match the general shape but still contain an invalid date or time. Check the calendar date and the allowed time fields rather than relying on appearance alone:
- Confirm the month is 01–12 and that the day exists in that month and year.
- Confirm the hour is 00–23 and the minute is 00–59.
- Seconds are normally 00–59. A value of 60 is allowed only for a leap second under the specification’s rules.
- If fractional seconds are included, RFC 3339 requires at least one digit, but does not require exactly three.
The full syntax and component rules are defined in RFC 3339.
Can these timestamps be sorted as text?
Fixed-width timestamp strings can sort lexically into chronological order when they use the same zone notation, represent the same zone, and have the same number of fractional-second digits. If offsets or fractional precision vary, normalize the timestamps before depending on a text sort. The ordering conditions are specified in RFC 3339.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Timestamp, not duration
This format identifies a date and time; it does not express an elapsed duration such as “five minutes.” The date-time fields locate the timestamp on a calendar and clock, while the fractional part adds subsecond precision.
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