How Excel stores dates
Excel stores a date as a serial number: 1 is 1 January 1900, and 46000 is in 2025. Time is the decimal part (0.5 = noon). This is why subtracting two dates gives a number of days, and why a date pasted as text (left-aligned in the cell) breaks every calculation. Use ISNUMBER(A2) to check: TRUE means a real date.
Age and seniority
DATEDIF is undocumented but present in every version. It returns complete years, months or days between two dates, which is exactly what HR needs for age or seniority.
=DATEDIF(B2;TODAY();"Y") & " years " & DATEDIF(B2;TODAY();"YM") & " months"Working days and deadlines
NETWORKDAYS counts business days between two dates, and WORKDAY adds business days to a start date. Pass a range of public holidays as the last argument so the result matches your real calendar. For countries with a Friday–Saturday weekend, use NETWORKDAYS.INTL with weekend code 7.
=WORKDAY(A2;10;Holidays!A:A)Month-end, quarters and grouping
EOMONTH(A2;0) returns the last day of the month, ideal for payment terms such as "30 days end of month": EOMONTH(A2;0)+30. To group by month in a report, use TEXT(A2;"yyyy-mm") as a sortable key, and =ROUNDUP(MONTH(A2)/3;0) for the quarter.
Converting text to real dates
When an export gives "2025-03-14" or "14/03/2025" as text, DATEVALUE depends on your regional settings and may swap day and month. Rebuild the date explicitly with DATE and text functions instead; the result is identical on every computer.
=DATE(RIGHT(A2;4);MID(A2;4;2);LEFT(A2;2))