Handling ambiguous local times during DST transitions in Erlang

Direct answer

Yes. Use the standard calendar module function calendar:local_time_to_universal_time_dst/1. It returns a list of every valid UTC interpretation for a local datetime: [] for a nonexistent spring-forward gap time, a single-element list when the local time is unambiguous, and a two-element list during a fall-back fold. This is the native, lossless way to detect and resolve the ambiguity without manual offset arithmetic.

One clarification: there is no standard OTP module named erlang_time. The built-in local-time conversion API lives in calendar. If you are actually calling a third-party library, please confirm its source, because the exact function names and semantics may differ.

Why a single-value API hides the fold

Any function that maps a bare local timestamp to one UTC value must silently pick one occurrence during a fold, because the local datetime alone does not contain the information needed to choose. That is a confirmed limitation of single-value conversion, not a defect unique to one library.

Confirmed OTP behavior: the _dst variant preserves both candidates. During a fall-back transition the two UTC values differ by the offset change (usually 3600 seconds); the earlier UTC corresponds to the pre-transition reading and the later UTC to the post-transition reading.

Recommended pattern for this case

1. Persist and transport timestamps as UTC or Unix time. Convert to local time only at the presentation edge, because UTC-to-local is always unambiguous.
2. When you must convert local input back to UTC—for example user input on a fall-back date—call calendar:local_time_to_universal_time_dst/1.
3. Match the two-element case explicitly and resolve it with domain knowledge such as an explicit first/second-occurrence flag, event ordering, or an accompanying UTC offset. Do not blindly take the first element.

Verification

On a host with a DST-observing time zone (for example TZ=Europe/Berlin), run:

calendar:local_time_to_universal_time_dst({{2026,10,25},{2,30,0}}).

Expect a two-element list whose UTC values differ by 3600 seconds. Run the same function on a spring-forward gap such as {{2026,3,29},{2,30,0}} and expect [], confirming gap detection. Compare with calendar:local_time_to_universal_time/1 to see which single candidate it returns.

Environment dependency

Erlang does not embed an IANA time zone database. The VM reads the host OS time zone at startup, so fold boundaries and offsets follow the host tzdata. Results are stable per host but can differ across machines running different tzdata versions. Do not hard-code DST rules or offsets.