About

Malformed time strings cause silent data corruption in scheduling systems, log analysis, and API integrations. A value like 13:72 passes naive string checks but represents no real moment. This validator parses input against 15 recognized clock-time patterns - including 24-hour (HH:MM:SS), 12-hour with AM/PM, ISO 8601 time components, and military notation - then applies range constraints: hours 0 - 23, minutes 0 - 59, seconds 0 - 59. It flags edge cases such as 12:00 AM (midnight, not noon) and 00:00 vs 24:00 boundary handling. The tool does not guess intent. It reports exactly what is wrong and why.

Limitations: this tool validates clock time only, not date-time combinations or timezone offsets (UTC+n). Fractional seconds are accepted up to 9 decimal places per ISO 8601. Pro tip: when processing user-submitted times, always normalize to 24-hour format before storage to eliminate AM/PM ambiguity.

Formulas

Validation proceeds in three stages: pattern matching, component extraction, and range checking.

valid(t) = match(t, P) ∧ range(h, m, s)

Where t is the input string, P is the set of recognized regex patterns, and the range function enforces:

0 ≤ h ≤ 23 ∧ 0 ≤ m ≤ 59 ∧ 0 ≤ s < 60

For 12-hour format, the hour constraint changes to 1 ≤ h ≤ 12. Conversion to 24-hour uses:

H₂₄ =

{

0 if h = 12 ∧ AM12 if h = 12 ∧ PMh if AMh + 12 if PM

Seconds since midnight for comparison:

S = H₂₄ × 3600 + m × 60 + s

Where H₂₄ = hour in 24-hour notation, m = minutes, s = seconds, S = total seconds from midnight (0 - 86399).

Reference Data

Format Name	Pattern	Example	Hours Range	Notes
24-hour (short)	HH:MM	14:30	00 - 23	Most common international format
24-hour (full)	HH:MM:SS	14:30:59	00 - 23	Includes seconds
24-hour (fractional)	HH:MM:SS.fff	14:30:59.123	00 - 23	Millisecond precision
12-hour (short)	hh:MM AP	2:30 PM	1 - 12	Leading zero optional
12-hour (full)	hh:MM:SS AP	02:30:59 AM	1 - 12	With seconds
12-hour (dot)	hh.MM ap	2.30 pm	1 - 12	Dot separator, case-insensitive
Military (HHMM)	HHMM	1430	00 - 23	No separator, 4 digits
Military (spoken)	HHMMh	1430h	00 - 23	Trailing h suffix
ISO 8601 (basic)	THH:MM:SS	T14:30:59	00 - 23	T prefix required
ISO 8601 (fractional)	THH:MM:SS,fff	T14:30:59,123	00 - 23	Comma as decimal (ISO standard)
ISO 8601 (compact)	THHMMSS	T143059	00 - 23	No separators
Dot-separated 24h	HH.MM	14.30	00 - 23	Common in some European locales
Dot-separated full	HH.MM.SS	14.30.59	00 - 23	Dot as separator throughout
Hours only	HH	14	0 - 23	Interpreted as HH:00:00
24:00 boundary	24:00	24:00:00	24 only	ISO 8601 end-of-day. 24:01 is invalid
Midnight zero	0:00	0:00	0	Single-digit hour accepted

Frequently Asked Questions

The 12-hour clock is cyclic with an offset. AM designates ante meridiem (before noon). Since 12:00 AM is midnight - the start of a new day - it maps to 00:00 in 24-hour notation. Conversely, 12:00 PM (noon) remains 12:00. This is the single most common source of scheduling errors in systems that convert between formats.

Yes, under ISO 8601. The value 24:00:00 represents the end of a calendar day and is equivalent to 00:00:00 of the following day. However, any value beyond 24:00:00 (e.g., 24:01) is invalid. This validator accepts 24:00 but flags a warning about its boundary nature.

Both 02:30 PM and 2:30 PM are accepted. The validator does not require leading zeros for single-digit hours in 12-hour mode but does require them in ISO 8601 and standard 24-hour formats where HH implies a two-digit field.

ISO 8601 permits arbitrary fractional precision for seconds. This validator accepts up to 9 decimal places (nanosecond resolution). Values like 14:30:59.123456789 are valid. The fractional part is parsed and displayed but does not affect the validity of the time itself - only the integer part of seconds must fall within 0 - 59.

No. Leap seconds (s = 60) are a real but extremely rare UTC adjustment. Most software systems, including POSIX time, do not support them. This validator treats 23:59:60 as invalid. If your system must handle leap seconds, additional date-context logic is required beyond clock-time validation.

ISO 8601 explicitly permits both comma and period as the decimal sign for fractional time components. The comma is actually the preferred separator in the standard. Both T14:30:59.5 and T14:30:59,5 are equally valid ISO 8601 representations.

Strictly, military time notation uses no separator: 1430 or 1430h. If you type 14:30, the validator classifies it as standard 24-hour format, not military. The distinction matters in protocol compliance but the underlying time value is identical.