HFS/HFS+ timestamp is a timestamp format (or, two variants of a timestamp format) associated with the HFS and HFS+ filesystems, and classic Macintosh computers. It appears in some file formats, for example ZIP extensible data field 0x334d (Info-ZIP Macintosh).
It may be called by other names, including Apple timestamp and Mac OS timestamp; though confusion may exist due to conflation of timestamp formats with timestamp epochs.
It is the number of (non-leap) seconds since the beginning of 1904. This date is the standard epoch for classic Macintosh computers.
It is usually stored as a 32-bit unsigned integer, in which case it supports dates from about 1904-01-01 to 2040-02-06.
It could be in universal time (UTC), or in local time (perhaps with a separate field for the timezone offset). It is normally in local time for HFS, and UTC for HFS+.
If it is in UTC, it differs from Unix time by 2082844800.