Excel serial number to date

Use it free in the browser when you are decoding a cell dump by hand, or call the API at $0.002 per successful request when you need the same contract inside automated pipelines.

What an Excel 1900 serial is and what the response contains

This converter accepts one required field, serial, as a finite number in the documented range, then returns the echo of the input serial, the Gregorian year, month number from one to twelve, day of month, hour, minute, and second in UT, a day_fraction measured from midnight UT, a YYYY-MM-DD date string, a readable month name, an iso_date echo, a datetime_ut string with a trailing Z, the system tag excel_1900, a boolean is_excel_leap_day that is true only for serial sixty, a leap_year_bug flag, the algorithm tag excel_1900_serial_with_leap_bug, and a short formula reminder of the day-count steps. There is no spreadsheet file upload, no timezone database lookup, and no random sampling: either the number is finite and in range and the fields are fully determined, or the request fails with invalid_input. Rejected cases include a missing serial, non-numeric tokens, NaN, positive or negative Infinity, values below one, and values at or above the Excel maximum through the thirty-first of December ninety-nine ninety-nine.

How serial is read and how the leap-year bug stays reproducible

Numeric strings with optional leading plus or minus signs and scientific notation are accepted when they parse as finite decimals; embedded spaces, hex prefixes, and empty strings are rejected with a clear message so a bad cell does not become a wrong calendar day. After parsing, the value must be finite and sit inside the inclusive lower bound of one and the exclusive upper bound that ends the day of thirty-one December ninety-nine ninety-nine. The preferred field is serial, but aliases excel_serial, value, n, and day are accepted so spreadsheets, query strings, and typed forms can reuse the same contract without renaming columns. From a valid serial the engine sets dayNum to the floor of the serial and day_fraction to the complementary fraction, maps dayNum through Excel’s three-hundred-sixty-six-day year nineteen hundred so that serial sixty remains the fictional leap day, then continues with real Gregorian day-of-year arithmetic from nineteen hundred one onward. Clock fields come from rounding the day fraction times eighty-six thousand four hundred to the nearest second so JSON goldens stay stable across runtimes. Pair the free browser run on this page with the priced API when you need the identical conversion inside an automated pipeline without re-implementing the Lotus leap quirk yourself.

Where Excel serial conversion fits in data work, audits, and software

On this page the work runs entirely in your browser; through the API each successful conversion costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a quick cell decode to a multi-million-row archive pipeline without changing the mathematical result.