How Excel stores dates as numbers
Behind the scenes a spreadsheet date is just a number. Excel counts days from an epoch, so 1 January 1900 is serial 1 in the default Windows system. The integer part is the day and any fractional part is the time of day. This tool converts in both directions and reproduces the quirks of Excel exactly, so the numbers it gives you match what the spreadsheet shows.
How it works
To convert a serial number to a date, the tool treats the value as days since 1899-12-30 in the 1900 system, an epoch chosen precisely so the count lines up with Excel for serial 61 and above. That offset accounts for Excel’s famous bug: it wrongly counts 29 February 1900, a date that never existed, at serial 60. The tool reports that the value is the phantom leap day when you land on serial 60, and adjusts serials in the 1 to 59 range so dates in early 1900 round-trip correctly.
44197 -> 2021-01-01
44197.75 -> 2021-01-01 18:00:00
60 (1900) -> phantom 1900-02-29The 1904 system counts from 1 January 1904 and has no leap-year bug, so its conversion is a straightforward day count from that epoch.
Example and notes
Pick the date system that matches your workbook. A given calendar date has a different serial number in the 1900 and 1904 systems, and mixing them is a common source of dates that are off by about four years. The fractional part is the time of day: 0.5 is noon and 0.75 is 18:00. If you only need whole dates, use integer serial numbers and ignore the fraction.