Weeks Between Dates Calculator

It parses both dates as ISO 8601 calendar dates (YYYY-MM-DD), subtracts them in milliseconds, and divides by 86,400,000 (24 x 60 x 60 x 1000) to get whole days. Weeks come from days divided by 7. Because the dates are parsed as local calendar dates rather than UTC instants, the count never shifts by a day across daylight saving transitions or different time zones.

Calendar days count every date in the range, weekends included. Business days count Mondays through Fridays only, so one business week equals five business days. Toggle "Count business days only" to use that five-day week. Public holidays are not deducted here; use the business days calculator when holidays matter.

Both readings are valid and the right choice depends on what you are counting. Leave the end date out (the default) when you are measuring a gap: Jan 1 to Jan 8 is 7 days because seven days pass between them. Tick "Include the end date" when you are counting calendar days touched, like vacation days or a pay period: that same span now reads as 8 days because both Jan 1 and Jan 8 are days you were off.

Decimal weeks (3.71) are easier to compare, average, or multiply, which is what spreadsheets and project plans want. Whole weeks plus remainder days (3 weeks and 5 days) reads more naturally and matches how people describe baby age, pregnancy progress, or vacation length. The two are the same number expressed two ways: 3.71 weeks is 25.97 days, which rounds to 26 days, which is 3 weeks and 5 days.

Calendar months range from 28 to 31 days, so a "5 month" pregnancy could span 140 to 153 days depending on which months you cross. Weeks are always 7 days, so dating a pregnancy in weeks gives the clinician a single precise number that maps cleanly onto fetal-development milestones (heartbeat at week 6, anatomy scan at week 20, full-term at week 37). Standard obstetric dating is 40 weeks from the last menstrual period, which is why every prenatal app counts weeks rather than months.