Methodology & Data Model

The 1970 Rule

As documented in theory.html, tzdb maintainers prioritize accuracy for civil timestamps after the Unix epoch (1970-01-01 00:00:00 UTC). Data prior to 1970 represents a best-effort compilation from historical gazetteers, government decrees, railroad timetables, and academic papers. Where regions merged after 1970, historical pre-1970 data lives in backzone and is explicitly lower quality.

Present-Day Geographic Boundaries

Time zones in tzdb represent present-day administrative territories with uniform civil timestamps since 1970. Historical borders changed continuously due to treaties, wars, and local ordinances. Displaying historical offsets across present-day boundary polygons follows the tzdb project's own design model.

Local Mean Time (LMT)

Before standard time conventions were codified in the late 19th and early 20th centuries, communities set clocks by local solar time at the meridian of their principal town. In tzdb, these appear as non-round offsets with abbreviation LMT. On our map and charts, LMT is rendered as a distinct grey hatch state, never as a modern civil offset.

Wall-Clock Gaps and Overlaps

When a region transitions forward (spring forward), local wall-clock times within the gap do not legally occur. When a region turns back (fall back), local wall-clock times within the overlap occur twice. Our database materializes these explicit states:

• gap: clock jumps forward, skipping a local interval.
• overlap: clock turns back, repeating a local interval.
• offset-only: standard time changes without daylight-saving gap/overlap.

Compilation with Fat Binaries

We compile tzdb source files using zic -b fat including backzone via PACKRATDATA=backzone PACKRATLIST=zone.tab. This ensures all transitions are fully materialized in the 64-bit TZif headers without truncation.