os.time and os.date behavior during DST transitions

Resolution of DST Ambiguity

Lua does not have an internal timezone database; it delegates all time calculations to the host operating system's C runtime (CRT) via mktime() and localtime(). Consequently, during the autumn DST fallback—where the same local time occurs twice—the resolution of ambiguous timestamps is platform-dependent.

Likely Behavior by Platform

• POSIX/Unix: The CRT typically resolves the ambiguous hour based on the current state of the TZ environment variable. In many implementations, if the time is ambiguous, the system defaults to the first occurrence (the DST period) or the second (the standard period) based on the specific C library version (glibc vs. musl).
• Windows: The resolution is governed by the Windows System Time API, which generally resolves ambiguous local times to the standard time (the second occurrence) once the transition has technically occurred.

Because Lua provides no built-in mechanism to specify the "fold" (whether to use the first or second occurrence of the hour), os.time() will return a single Unix timestamp based on the OS's internal preference for that specific local time input.

Verifying DST Offset Without External Libraries

You can verify if the current time is reflecting a DST offset by using os.date with specific format strings. This allows you to check the numeric offset or the timezone abbreviation provided by the OS.

-- Check the numeric UTC offset (e.g., -0700 or -0800)
print("Numeric Offset: " .. os.date("%z"))

-- Check the timezone abbreviation (e.g., PDT vs PST)
print("Timezone Name: " .. os.date("%Z"))

Verification Steps

1. Capture Offset: Run os.date("%z") to see the current offset from UTC.
2. Compare: Compare this value against your region's known Standard Time offset. If the offset is smaller (closer to UTC) during summer months, DST is active.
3. Validate Transition: On POSIX systems, you can simulate a transition by changing the TZ environment variable before launching the Lua interpreter to see how os.time() resolves the same local hour under different rules.

Diagnostic Note: To provide a more precise prediction of your specific resolution, please specify the host operating system and the C runtime version (e.g., Ubuntu 22.04 with glibc) being used.