Revert Excel Serial Dates Back to Intended Day.Month Numerics
Detects values imported from spreadsheets that were auto-converted to Excel date serials and reconstructs the originally intended day.month decimals (for example, '30.3' that Excel displayed as '30/03/2025'). The functions work in a vectorized manner, preserve non-serial values, and support both the 1900 and 1904 date systems.