Excel Serial Date Converter

Formula

How this is calculated

Spreadsheets store dates as a single number: the count of days since an epoch, with the fractional part encoding the time of day. In the default 1900 system the epoch is 30 December 1899, so the serial of a date is round((date − Date.UTC(1899,11,30)) / 86,400,000). This offset deliberately bakes in Excel’s well-known fictitious 29 February 1900 leap day, which makes serials match Excel for all real-world (post-February-1900) dates. The 1904 system, used by old Macs and selectable in Google Sheets, counts from 1 January 1904 instead; its serials are exactly 1462 days lower than the 1900 system for the same date.

The Direction selector chooses the conversion. For serial-to-date, the tool reads the Serial number, adds the optional Time fraction, then rebuilds the calendar date and clock time. For date-to-serial, it parses the ISO date, rounds to the nearest whole day from the chosen epoch, and adds the time fraction. The Time fraction input is a value from 0 to 1: 0.25 is 06:00, 0.5 is noon, and 0.75 is 18:00, since one full day equals 1.0.

Results are computed in UTC to avoid daylight-saving and timezone drift, and the millisecond timestamp is rounded to the nearest second before formatting. The outputs show the serial number, the calendar date (YYYY-MM-DD), the time of day, the raw day fraction, and a combined ISO 8601 string. Note that the first 59 days of 1900 are the only dates where Excel’s phantom leap day causes a one-day discrepancy versus true calendar arithmetic.