A Practical Guide to Calendar Math
These notes describe the rules Datefold Studio actually uses. They are not a history of every calendar ever printed.
01 — Calendar Days
Calendar days include weekdays and weekends. If you add 10 calendar days to a Wednesday you will pass through a weekend and land on a Saturday. Example: 2026-03-04 (Wednesday) + 10 days = 2026-03-14 (Saturday). Use the add days calculator when weekends should count.
02 — Inclusive Counting
Elapsed days measure the open gap. Inclusive counting counts both named dates. Monday to Friday elapsed is 4; inclusive is 5. Same-day elapsed is 0; inclusive is 1. Example: 2026-01-01 to 2026-01-03 elapsed 2, inclusive 3. Toggle this on the difference calculator.
03 — Full Weeks
Seven consecutive calendar days make one elapsed week here, regardless of Monday or Sunday. 15 days = 2 full weeks and 1 extra day. That is not an ISO week-numbering system. See the week breakdown on the date difference result.
04 — Weekdays
Weekdays are Monday through Friday. Saturday and Sunday are weekend dates. This site does not treat Friday sunset or Sunday as special cases. Count them with the weekday counter.
05 — Business Days and Holidays
Business days equal weekdays. The calculator does not load a holiday table. A Wednesday holiday still counts as a weekday. Example: a range that includes 2026-01-01 still treats that Thursday as a business day. Use business days only as a weekday sketch.
06 — Leap Years
Divisible by 4, except centuries, unless divisible by 400. 2000 leaps; 1900 does not; 2024 leaps; 2100 does not. February 29, 2024 exists; February 29, 2025 does not. Check a date in the weekday finder.
07 — Month Lengths
Months are 28, 29, 30, or 31 days long. You cannot treat a month as 30 days and stay aligned with calendar pages. January 31 plus one month clamps to February’s last day. That rule lives in the months and years calculator.
08 — Daylight Saving Time
Springing forward or falling back changes local clocks, not the sequence of calendar dates. Datefold Studio parses year-month-day and uses UTC ordinals so a DST night cannot turn one calendar step into zero or two days. The difference calculator is built on that rule.