What is Weeks Between Two Dates Calculator?
weeks between dates, pregnancy weeks This tool runs entirely in your browser. No data is sent to any server, and no signup is needed.
Weeks Between Two Dates Calculator
Exact number of weeks between two dates. Useful for pregnancy tracking, project planning, and rolling-period calculations.
About the weeks-between-dates calculator
This tool returns the exact number of weeks between two calendar dates, expressed three ways at once: as a precise decimal (for example 12.86 weeks), as whole weeks plus leftover days (12 weeks + 6 days), and as a total day count for cross-checking. That triple readout is the standard helper for pregnancy tracking (where clinicians count in completed weeks plus days), agile sprint planning (where managers care about decimal sprints), contract milestones (where lawyers care about whole-week thresholds), and quarter-by-quarter reporting (where finance teams reconcile 13-week blocks).
Unlike a simple day-counter, this calculator was built around the asymmetric way humans actually quote week ranges. A pregnant person at day 90 is "12 weeks and 6 days", not "12.86 weeks". A sprint that ran from January 6 to January 19 was "2 sprints", not "1.86 sprints". The tool keeps both forms visible so you can grab the one your audience expects without re-doing the math by hand.
How it works
The calculator parses both dates as midnight UTC, subtracts the timestamps in milliseconds, converts to days, and divides by 7. Leap years are handled implicitly because Date arithmetic operates on absolute timestamps rather than month-day counters: a year boundary, a leap day, or a 28-day February all just become part of the underlying epoch difference.
days = round((end_ms - start_ms) / 86,400,000) weeks = days / 7 whole = floor(weeks) extra = days mod 7
- Round at days first: protects against daylight-saving 23 or 25 hour days. Without the round, a March-to-April range could come out as 30.96 days and silently lose a week boundary.
- Decimal output: precise to two places; useful for fractional sprints, partial-week payroll, and contract calculations where every day matters.
- Whole + extra: matches how most contracts and pregnancy charts read, including the "X weeks Y days" form ultrasound reports use for gestational age.
- Order-independent: switching start and end produces the same magnitude; the tool reports the absolute difference, not a signed one.
Worked example
Pregnancy due date estimate: last menstrual period (LMP) on 2026-01-15, today is 2026-05-28.
- Start: 2026-01-15 (LMP).
- End: 2026-05-28 (today).
- Days difference: 133 days.
- Weeks: 133 / 7 = 19.00 weeks.
- Whole + extra: 19 weeks + 0 days.
- Term reference: full term at 40 weeks from LMP, so 21 weeks to go.
Common week-based timeframes
| Context | Length | Notes |
|---|---|---|
| Pregnancy (full term) | 40 weeks from LMP | 38 weeks from conception; preterm under 37 |
| Pregnancy (1st trimester) | Weeks 1 to 13 | Highest miscarriage risk; major organ formation |
| Pregnancy (2nd trimester) | Weeks 14 to 27 | Anatomy scan ~20 weeks; viability ~24 weeks |
| Pregnancy (3rd trimester) | Weeks 28 to 40 | Glucose test 24 to 28 weeks; Tdap 27 to 36 |
| Agile sprint | 2 weeks | Most common; 1, 3, 4 weeks also used |
| SAFe Program Increment | 10 weeks | Usually 5 sprints of 2 weeks each |
| Fiscal quarter | ~13 weeks | 4 quarters x 13 = 52 weeks/year |
| School term (UK) | ~13 weeks | 3 terms per year, plus holidays |
| Mortgage early-pay | 52 vs 26 payments | Bi-weekly = 13 monthly payments per year |
Common pitfalls
- Timezone shifts. A date entered as "2026-03-08" in Pacific time and parsed as UTC can drift by a day. The tool uses midnight UTC for both ends to avoid this.
- Daylight saving doesn't add a week. DST changes shift one day by an hour, but the round-to-days step absorbs that. Don't subtract hours.
- End-inclusive vs end-exclusive. Contracts may count the end date or not. This tool reports the exclusive difference; add 1 day for inclusive counts.
- Pregnancy week confusion. "I'm 12 weeks pregnant" typically means in the 13th week, between days 84 and 90 from LMP. The week count restarts each 7 days, so 12 weeks 6 days is still "12 weeks".
- Sprint cadence drift. A 2-week sprint maps cleanly to 26 sprints per year only if the team treats holidays as zero-week. Add buffer for company holidays.
- ISO vs US week numbering. Week 1 of 2026 starts on a different date in ISO vs US conventions; do not mix the two within a project plan.
Related tools and glossary
Frequently asked questions
How long is a pregnancy in weeks?
Term is 40 weeks measured from the first day of the last menstrual period (LMP). Measured from conception, term is about 38 weeks because ovulation occurs roughly 2 weeks into the LMP cycle. Preterm is under 37 weeks; postterm is over 42 weeks.
How many weeks are in an agile sprint or quarter?
Standard agile sprints are 2 weeks, so 6 months equals 26 weeks or 13 sprints. SAFe Program Increments (PIs) are 10 weeks, or 5 sprints. A fiscal quarter is about 13 weeks, and four quarters of 13 weeks total 52 weeks plus one or two leap-smoothing days.
How do I handle partial weeks?
This calculator returns decimal weeks (for example 12.86 weeks). Contracts typically round one of three ways: round up (any partial week counts as a full week), round down (only complete weeks count), or use bankers rounding (round half to even). Check your contract before applying a rule.
What is the difference between ISO 8601 weeks and US calendar weeks?
ISO 8601 weeks start on Monday and use the ISO week-numbering year, so week 1 contains the first Thursday of the calendar year. US and Canadian calendars typically start the week on Sunday and number from January 1. For payroll, financial reporting, or international standards, use ISO 8601.
Why does my answer differ by one day from a competitor calculator?
Different tools choose different rounding boundaries. Some snap to local time and absorb daylight-saving shifts; others use Julian-day arithmetic that ignores DST entirely. The 3Tej tool parses both dates as midnight UTC and rounds at the day level, which produces stable, timezone-neutral output. If your other source counts the end date inclusively (so March 1 to March 8 counts as 8 days, not 7), add one day before dividing by 7.
Sources
- American College of Obstetricians and Gynecologists (ACOG), pregnancy dating guidelines.
- ISO 8601:2019, Date and time, Representations for information interchange.
- Scaled Agile Framework (SAFe) 6.0, Program Increment definition.