Date Difference: Days Between Dates & Date Arithmetic

Convention

This span

Best for

Plain days

365

Contracts, interest, billing

Calendar Y-M-D

1y 0m 0d

Age, tenure, anniversaries

Workdays

261

Project planning (excl. holidays)

Key Takeaways

Basic rule: difference = end date − start date; Jan 15 → Mar 20, 2025 = 64 days

Inclusive: "from X to Y inclusive" = difference + 1 — the off-by-one classic

Month lengths: 31/28(or 29)/31/30/31/30/31/31/30/31/30/31 — knuckle method

Banking 30/360: finance often pretends every month has 30 days, year 360

Core answer: to find the days between two dates, add up each month's days. Example: 2026-01-15 to 2026-04-20 = 16 remaining in Jan (31−15) + 28 (Feb) + 31 (Mar) + 20 (Apr) = 95 days. "Days between" excludes the start date; add 1 if you need both ends included.

The basic algorithm

Days left in the start month = month length − start day
Add every full month in between
Add the end month's day number

Convention: difference = end − start (start date excluded). Contracts and interest calculations often count both ends — add 1 there.

Month lengths and leap years

Month	Days	Month	Days
Jan	31	Jul	31
Feb	28 (29 leap)	Aug	31
Mar	31	Sep	30
Apr	30	Oct	31
May	31	Nov	30
Jun	30	Dec	31

Leap-year rule: divisible by 4 but not 100, unless also by 400. 2024 and 2028 are leap years; 2100 is not; 2000 was.

Cross-month example

2026-03-10 to 2026-06-05 (2026 not a leap year):

March remaining: 31 − 10 = 21
April: 30, May: 31
June: 5
Total: 21+30+31+5 = 87 days

Cross-year example

2025-11-20 to 2026-03-01:

Nov 2025 remaining: 30 − 20 = 10
December: 31
Jan 31 + Feb 28 (non-leap) + Mar 1
Total: 10+31+31+28+1 = 101 days

Estimating workdays

Rough: total days × 5/7 (87 days ≈ 62 workdays)
Exact: subtract weekends week by week, then public holidays
China has ~11-13 statutory holidays per year (after shift adjustments); annual leave is separate
For "N working days" in a contract, always confirm whether shifted-working weekends count

Common mistakes

"Inclusive or exclusive — who cares": one day is real money in interest and penalty clauses. Banks typically "count the head, not the tail" (deposit day accrues, withdrawal day doesn't).
"Treat every month as 30 days": the 30.42-day average works only for annualized estimates. Exact math must use real month lengths — February's 28/29 compounds errors.
"A year is 365 days": spanning 2024/2028, a "one-year" period may be 366 days.

Frequently Asked Questions

How do I count days between two dates?

Subtract: end − start. Jan 15 to Mar 20, 2025: remaining Jan (31−15 = 16) + Feb 28 + 20 = 64 days. For long spans, count by months then adjust: Mar 20, 2025 → Sep 8, 2025 = 11 (Mar) + 30 + 31 + 30 + 31 + 31 + 8 = 172... trust the calculator beyond a month or two — month-length variance makes mental arithmetic error-prone.

When do I add 1 to the count?

Exclusive difference (end − start) counts midnights passed; inclusive counting adds 1. "Stay from the 10th to the 15th": exclusive = 5 nights, inclusive = 6 days. Contracts, hotel nights and leave policies each pick a convention — the off-by-one between them is a real source of disputes. When it matters, state the convention explicitly: "inclusive of both endpoints."

How do I remember month lengths?

Knuckle method: make a fist, count months across knuckles and gaps — knuckle = 31 days, gap = 30 (February the special gap). July and August are adjacent knuckles (both 31). The sequence: 31, 28/29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31. Only February varies; all others are fixed.

How do leap years affect date math?

Leap rule: divisible by 4, except centuries unless divisible by 400 — 2024 leap, 2100 not, 2000 leap. Feb 29 shifts every date after it by a day within that year. Spans crossing Feb 29 in a leap year get +1 day. A "year" is 365.2422 days — the reason for leap years and the 400-year exception (0.2422 × 400 ≈ 97 leap days needed per 400 years).

What is the 30/360 banking convention?

Bond and loan interest often assumes 30-day months and 360-day years: interest = principal × rate × days/360 with each month counted as 30. Alternatives: actual/365 (real days over 365) and actual/360 ("banker's year" — real days over 360, which slightly overcharges). Same nominal rate, different real interest — check the day-count convention in loan documents; actual/360 costs ~1.4% more than actual/365.

What mistakes happen in date arithmetic?

(1) Inclusive/exclusive off-by-one — the champion; (2) forgetting February's 28/29 switch; (3) "one month later" from Jan 31 → Mar 3 or Feb 28 depending on convention (no universal answer); (4) assuming a year = 52 weeks (it is 52 weeks + 1 day, +2 in leap years — weekdays drift); (5) mixing 30/360 with actual-day interest; (6) time zones on date boundaries — a Monday 23:30 UTC event is already Tuesday in Beijing.