Day 16: Date & Time Functions

TODAY() returns today's date (updates daily). NOW() returns current date and time. YEAR(date), MONTH(date), DAY(date) extract year, month, day as numbers. WEEKDAY(date,2) returns 1=Monday to 7=Sunday (mode 2 is ISO standard). WEEKNUM(date) returns the week number of the year. EOMONTH(date,0) returns the last day of the month. These are the building blocks for all date-based analysis.

NETWORKDAYS(start_date, end_date, [holidays]) counts working days (Monday–Friday) between two dates, inclusive of both. An optional holidays range excludes those dates. NETWORKDAYS.INTL allows custom weekend definitions — e.g. for businesses that work on Saturdays: NETWORKDAYS.INTL(start,end,11) where 11 means only Sunday is the weekend. This is commonly used for SLA calculations, delivery time analysis, and project duration.

DATEDIF(start_date, end_date, unit) calculates the difference between two dates in years, months, or days. Units: "Y"=complete years, "M"=complete months, "D"=total days, "YM"=months ignoring years, "MD"=days ignoring months and years. Classic use: =DATEDIF(BirthDate, TODAY(), "Y") calculates exact age in complete years. DATEDIF is intentionally undocumented in Excel (inherited from Lotus 1-2-3) — it works but Microsoft does not officially support it. Use YEARFRAC for year fractions.

Three approaches: (1) =TEXT(A2,"MMMM") returns full month name ("August"). (2) =TEXT(A2,"MMM") returns abbreviated name ("Aug"). (3) =MONTH(A2) returns the month number (8) — use this when you need to sort or calculate, not display. Important: TEXT returns text, so you cannot sort month names alphabetically with TEXT — they sort alphabetically (April, August...) not chronologically. Use MONTH number for sorting, TEXT for display.

WORKDAY(start_date, days, [holidays]) returns a date that is exactly N working days after (positive) or before (negative) the start date. =WORKDAY(TODAY(), 5) returns the date 5 business days from today. =WORKDAY(TODAY(), -10) returns 10 business days ago. WORKDAY.INTL allows custom weekends. Useful for: calculating when a deliverable is due given a business-days SLA, finding the date N days back for reporting windows.