Glossary

Epoch — the zero point a timestamp counts from (Unix 1970-01-01, FILETIME 1601-01-01, Cocoa 2001-01-01, …). See format reference.

Tick / unit — the smallest increment a format counts in: seconds, milliseconds, microseconds, 100-nanoseconds (FILETIME/.NET), or nanoseconds. See Precision.

POSIX time — seconds since 1970 counting every day as exactly 86 400 s, i.e. leap-second-ignoring. Not the same as UTC around a leap second. timeglyph's internal spine is PosixNs for this reason.

UTC / TAI / GPS / UT1 — the time scales. UTC is civil time (with leap seconds); TAI is continuous atomic time; GPS is continuous from 1980; UT1 tracks Earth's rotation.

Leap second — a ±1 s adjustment to UTC (a real 23:59:60) to track Earth's rotation, decided by the IERS. 27 inserted since 1972; being phased out by ~2035.

Leap smear — spreading a leap second over a window (Google/AWS/Meta) so no second repeats; invisible in a raw timestamp.

Julian / Gregorian — the calendars. The 1582 Gregorian reform dropped 10 days; the leap-year rules differ at century boundaries.

Proleptic Gregorian — the Gregorian calendar extended backward before 1582; what ISO 8601 and most software (incl. jiff) compute on.

Astronomical year numbering — includes a year 0 (= 1 BC); used by ISO 8601.

Julian Day (JD) — a continuous day count from noon, 24 Nov 4714 BC; JD 2440587.5 = the Unix epoch. MJD = JD − 2400000.5.

MACB — the four NTFS/forensic timestamps: Modified, Accessed, Created (born), MFT-changed. See NTFS SI vs FN.

FILETIME — Windows 64-bit count of 100-ns intervals since 1601 UTC.

Snowflake — a 64-bit ID with a millisecond timestamp in its high bits (Twitter/X, Discord). See identifiers.

BCD (binary-coded decimal) — each decimal digit stored in 4 bits; used by the MC146818 RTC. 0x59 = 59, not 89.

Era — a rollover generation: NTP's 32-bit seconds wrap into era 1 in 2036; GPS's 10-bit week wraps every 1024 weeks. See rollovers.

Sentinel — a magic value (0, all-ones, 0x7FFFFFFFFFFFFFFF) meaning unset/never/error, not a real instant. See sentinel values.

Encoding vs format — format is the semantics (epoch/unit); encoding is the presentation (radix/endianness/width/packing). See input conventions.

Tier-1 validation — a check where an independent third party authored both the input and the answer (a spec worked example or an independent oracle tool). See Validation.