The Macintosh epoch

When the original Macintosh shipped, its clock started at midnight on January 1, 1904. A round number that was exactly 66 years before the Unix epoch. That's the offset: 2082844800 seconds (1904 → 1970).

HFS+ → Unix: unix = hfs - 2082844800.
Unix → HFS+: hfs = unix + 2082844800.

You will meet it in hfs.creation_date / hfs.modified_date fields from forensic tools, in .AppleDouble metadata, and in the resource forks of very old archives.

From the 1904 zero point to today

0 → January 1, 1904 00:00:00 UTC, the very beginning of Mac time.
2082844800 → January 1, 1970 00:00:00 UTC, the two epochs meet.
3868905600 → August 7, 2026 00:00:00 UTC.

Decoding HFS+ seconds in Python, Go, and JavaScript

Python: datetime(1904, 1, 1, tzinfo=timezone.utc) + timedelta(seconds=hfs).
Go: time.Unix(hfs-2082844800, 0).UTC().
JavaScript: new Date((hfs - 2082844800) * 1000).

Gotchas: the 2040 wrap, local-time history, and HFS vs HFS+

32-bit wrap, classic HFS stored timestamps as unsigned 32-bit seconds, so the format runs out on February 6, 2040 (max 4294967295).
Local vs UTC, early Macs stored local time; HFS+ and this converter treat values as UTC. Values written before ~1990 may be shifted by your machine's historical offset.
HFS vs HFS+, both use the 1904 epoch; HFS+ simply extended the fields to 32-bit seconds.
Not the Apple epoch, modern Apple platforms use Unix time. Only legacy formats use 1904.

Related tools

For the other pre-1970 era used on classic platforms, see the Excel date converter (days since 1900) or the NTP timestamp converter (seconds since 1900).

Mac HFS+ Timestamp Converter. Seconds Since 1904