Excel Date Converter. OADate Serial Numbers

How Excel dates work

Excel's 1900 date system stores 2026-08-07 00:00 as 46241, the number of days since the system's zero point. The same format, called the OLE Automation date (OADate), is used by COM, VBA, python-pptx, and many databases when they talk about dates:

• Serial → Unix: unix_ms = (serial - 25569) * 86400000.
• Unix → Serial: serial = unix_ms / 86400000 + 25569.
• Time of day, the fractional part: 0.5 is noon, 0.25 is 06:00.

The zero point is December 30, 1899, chosen so the leap-day bug below stays invisible for every date Excel actually supports.

Serial dates decoded

• 2 → January 1, 1900.
• 25569 → January 1, 1970 00:00:00 UTC, the Unix epoch, in serial days.
• 46241.5 → August 7, 2026 12:00:00 UTC.

Serial dates in Python, JavaScript, and C#

• Python: datetime(1899, 12, 30, tzinfo=timezone.utc) + timedelta(days=serial).
• JavaScript: new Date((serial - 25569) * 86400000).
• C#: DateTime.FromOADate(46241.5) and DateTime.Now.ToOADate().

The 1900 leap-year bug

Excel's serial 60 is 1900-02-29, a day that never existed, because Excel treats 1900 as a leap year (Lotus 1-2-3 did, and Excel kept the bug for compatibility). The OADate system sidesteps it by starting one day earlier, on 1899-12-30, so:

• Serials 1–59 (Jan 1 – Feb 28, 1900) match the same dates in both systems.
• Serial 60 is the fake day 1900-02-29; because the OADate zero point sits at 1899-12-30, this converter maps serial 60 to the real date 1900-02-28. Excel itself displays 1900-02-29 for the same serial.
• From serial 61 (1900-03-01) onward, both systems agree with the real calendar.

Gotchas: fractions, negative serials, and the 1904 system

• Fraction = time. 46241 is midnight and 46241.99999 is 23:59:59; the decimal part is a fraction of a day, not hours.
• Negative serials, dates before 1899-12-30 produce negative numbers that Excel displays inconsistently; this converter handles them mathematically.
• 1904 system, classic Mac Excel used a 1904-based system (serial 0 = 1904-01-01; only the 1900-based system starts at 1899-12-30). Add 1462 days to convert a serial between the two systems.