How to use the Excel Date Serial Number Converter
- 1. Choose a directionPick serial number to date, or date to serial number.
- 2. Enter the valueType the serial (decimals allowed) or pick a date and optional time.
- 3. Read the resultSee the date, ISO format, time and a DATE() formula to rebuild it.
How Excel stores dates
Excel stores a date as the number of days since a start date, and the time as a fraction of a day. In the default 1900 system, serial 1 is 1 January 1900. Noon is .5, so 45000.5 is noon on the same day as 45000.
For serials from 61 upward the rule is simple: date = 30 December 1899 + serial days. So 45000 is Wednesday, 15 March 2023, and going the other way, 1 January 2024 is serial 45292. Data exported from databases or CSV files often arrives as these numbers, and this tool turns them back into dates.
The 1900 leap year bug and the 1904 system
Excel treats 1900 as a leap year even though it was not. Serial 60 is shown as 29 February 1900, a date that never existed. This copies a bug from Lotus 1-2-3 so old spreadsheets kept working. The converter flags serial 60, and counts serials 1 to 59 from 31 December 1899 so early 1900 dates still match Excel.
Some old Mac workbooks use the 1904 system, where serial 0 is 1 January 1904. The same date has a serial 1,462 lower in that system. If dates from a colleague's file look four years off, check File › Options › Advanced › Use 1904 date system.
- In Excel,
=DATE(2023,3,15)returns 45000 when formatted as a number. - In Python pandas, use
pd.to_datetime(45000, unit='D', origin='1899-12-30').
Need date maths too? Try the moving average calculator for time series.
Frequently asked questions
Why does my CSV show 45000 instead of a date?
The column was saved as a number. Convert it here, or format the cells as Date in Excel.
Does Google Sheets use the same numbers?
Yes. Google Sheets uses 30 December 1899 as day 0, so serials from 61 upward match Excel exactly.
Is the Excel Date Serial Number Converter free?
Yes, the Excel Date Serial Number Converter is free. The first time you see your first conversion, you are asked for your name and WhatsApp number once. After that, this and every other Uncodemy tool opens straight away without the form.