fromExcelSerial

Signature

fromExcelSerial(value: number, options?: { system?: ExcelDateSystem; }): string

Convert an Excel day serial back to an ISO 8601 instant string.

The serial is days with the time of day as the fraction, read as a UTC wall clock.
Serial 60 in the 1900 system returns "" — it is the phantom 29 February 1900, a date that never existed. Lotus 1-2-3 treated 1900 as a leap year and Excel keeps the bug for compatibility, so the whole serial day [60, 61) is a hole: 59 is 1900-02-28 and 61 is 1900-03-01. Mapping 60 to either neighbour would put every earlier date one day out.
In the 1904 system serial 60 is a real 1904-03-01.
Accepted range is Excel’s own: serial 1 (1900-01-01) to just under 2958466 (the end of 9999-12-31) in the 1900 system, serial 0 to just under 2957004 in the 1904 system. Serial 0 in the 1900 system is Excel’s “January 0, 1900” placeholder, not a date.
Rounds to the nearest millisecond. A serial is a double, whose resolution is about a microsecond near 2024 and about 40 µs near 9999 — finer than a millisecond nowhere in the range, so rounding there is what makes the round trip through toExcelSerial exact.
Returns "" on invalid input.

Parameter	Type	Description
`value`	`number`	Excel day serial (number, may carry a fractional time of day)

options

Which Excel date system the serial is in

Option	Type	Default	Description
`system?`	`"1900"` \| `"1904"` (`ExcelDateSystem`)	`"1900"`	Which Excel date system the serial is counted in. `"1900"` counts serial 1 as 1900-01-01 and has the phantom serial 60; `"1904"` is the legacy Mac system, whose serial 0 is 1904-01-01 and which has no phantom day. The same instant is exactly 1462 lower in the 1904 system.

ISO 8601 instant string (UTC), or "" on invalid input

import { fromExcelSerial } from "@northguild/gmt/precision/convert";

fromExcelSerial(1) // "1900-01-01T00:00:00Z" — Excel's first serial