Theory
How many days overdue?
A supplier's payment was due on an invoice dated 10 June. Meera wants to know, today, how many days overdue it is. On paper she would count on a calendar, awkward across month ends.
In Excel she types =TODAY()-DATE(2026,6,10) and gets the answer as a plain number.
How can you subtract two dates and get a count of days? Because of a secret about how Excel stores dates, one that makes all date maths suddenly simple. That secret is this lesson.
Theory
Dates are day-numbers in disguise
Imagine a giant calendar where every day since 1 January 1900 has a serial number: day 1, day 2, day 3... all the way to today (around day 46,000). Excel stores dates exactly like this, as plain numbers wearing a date costume. Once you know a date is really a number, subtracting two dates to count the days between them stops being magic: it is just number minus number.
Theory
The date and time toolkit
Because dates are serial numbers, these functions all work naturally:
- TODAY(): current date. NOW(): current date and time.
- DATE(year, month, day): builds a date from parts.
- DAY / MONTH / YEAR: pull a piece out of a date.
- HOUR / MINUTE / SECOND: pull a piece out of a time.
- WEEKDAY(date): returns 1 to 7 for which day of the week it is.
- DAYS360(start, end): days between two dates assuming a 360-day (12 x 30) year, used in interest and finance.
Quiz
Why can Meera write =B2-A2 to get the number of days between two dates in A2 and B2?
- Excel stores dates as serial numbers, so subtracting them gives a day count
- Excel has a hidden calendar-counting engine for dates only
- It only works if both dates are in the same month
- It does not work; you must use DAYS360 always
Show the answer
Excel stores dates as serial numbers, so subtracting them gives a day count
Every date is stored as a serial number (days since 1 Jan 1900), so B2 - A2 is just one number minus another, giving the days between. No special engine, no same-month rule. This "dates are numbers" insight is the key fact examiners probe, and it makes every date calculation intuitive.
Think first
Find the busiest day
Meera has a column of sale dates and wants to know which WEEKDAY (Mon-Sun) is busiest. Which function turns each date into a day-of-week, and what kind of value does it return?
Show the answer
WEEKDAY(date). It returns a number from 1 to 7 representing the day of the week (by default 1 = Sunday). Meera can then count how many sales fell on each weekday number to spot her busiest day. The point: WEEKDAY gives a number, not the word 'Monday', so you use it for counting and comparing, not display.
Watch out
Where marks leak
Thinking dates are stored as text, they are serial numbers, which is why arithmetic works. Confusing TODAY() (date only) with NOW() (date and time). Forgetting that WEEKDAY returns a number (1-7), not a day name. And mixing DAYS360 (assumes 30-day months, for finance) with a plain date subtraction (actual calendar days), exams ask why a finance calculation uses DAYS360 (answer: standardised 360-day interest year).
Theory
Numbers in costume, again
Notice the pattern repeating: a char is a number in costume (BCA104), a percentage is a decimal in costume (last unit), and now a date is a serial number in costume. Computers store almost everything as numbers and dress them up for humans. Spot this and half of computing stops being mysterious. Next: turning Meera's numbers into charts that reveal trends at a glance.
Summary
Key takeaways
- Excel stores dates as serial numbers (days since 1 Jan 1900), so date2 - date1 = days between.
- TODAY() gives the date; NOW() gives date and time.
- DATE builds a date; DAY/MONTH/YEAR and HOUR/MINUTE/SECOND extract parts.
- WEEKDAY returns a number 1-7 for the day of the week (great for finding busiest days).
- DAYS360 counts days on a 360-day year for financial/interest calculations.
- Memory hook: a date is a day-number in a date costume.