Convert Excel serial dates in the 1900 or 1904 system to ISO UTC and back.
Type or paste on the left — the result updates as you type.
Rejects fictitious serial day 60 in the 1900 system. Fractions represent time of day; conversion rounds to milliseconds and assumes UTC.
Up to 100,000 UTF-16 units per input and 1,000,000 per result. See the supported formats and examples below.
Tool page updated: 2026-09-27
