logohsb.horse

DST Ambiguous and Missing Time Checker

Classify a local time in an IANA time zone as unique, ambiguous, or nonexistent.

Check fall-back overlaps where one clock time occurs twice and spring-forward gaps where a local time never occurs.

The check runs entirely in this browser using its built-in time-zone data.

Local date
Local time
IANA time zone

Result

Enter a local date, time, and time zone.

    How to use

    1. 1

      Enter local time

      Enter the wall-clock date and time used by a schedule or job.

    2. 2

      Choose a time zone

      Use an IANA name such as America/New_York.

    3. 3

      Inspect candidates

      Change a zero-candidate time or explicitly choose an offset for two candidates.

    Scope

    Results depend on the Intl time-zone data built into this browser. For critical schedules, verify against the actual runtime because laws and data versions can differ.

    FAQ

    What is an ambiguous local time?
    During fall back, one wall-clock time occurs twice and maps to two UTC instants.
    What is a nonexistent local time?
    During spring forward, the clock skips a range that maps to no UTC instant.
    Can I enter a UTC offset instead?
    No. An IANA zone is needed to apply the region's transition rules.