How accurate is this conception date calculator?

Calculations are precise to the day, accounting for leap years and varying month lengths. Results update in real-time as you enter dates.

Does it account for time zones?

Date calculations are based on calendar dates. For time-sensitive applications, consider your local time zone when interpreting results.

Can I save my calculations?

You can bookmark the page or copy the results. The tool processes everything locally in your browser for privacy.

Conception Date Calculator

Trimester	Weeks	What happens
1	0–13	Organ formation; the highest-risk period
2	14–27	Growth; the anatomy scan falls around week 20
3	28–40+	Maturation; viability improves week by week

Perform date calculations quickly and accurately with this conception date calculator.

How It Works

Input your dates to calculate:

Duration between dates
Future or past dates
Business days
Weekends and holidays

Practical Applications

Project timeline planning
Age calculations for forms
Historical date research
Scheduling and booking

Estimating a Conception Date

Conception dates are estimated backwards from a due date or a last menstrual period, and the estimate carries real uncertainty.

Method	Basis	Typical accuracy
Naegele's rule	LMP + 280 days	±2 weeks
Ovulation estimate	LMP + 14 days	±3 days for regular cycles
First-trimester ultrasound	Crown-rump length	±5 days
Known IVF transfer date	Direct	Exact

Two facts drive the imprecision. Pregnancy is dated from the last menstrual period, not from conception, so "6 weeks pregnant" means roughly 4 weeks since conception. And ovulation does not reliably occur on day 14 — cycle length varies between women and between cycles.

Sperm survives up to five days in the reproductive tract, so intercourse and conception can be days apart. Any date produced here is an estimate; a dating ultrasound is the clinical answer.

The Traps in Date Arithmetic

Months are not 30 days. Adding one month to 31 January has no correct answer, and every library picks differently — 28 February, 2 March or an error. Decide which your business needs before the edge case decides for you.

Days are not always 86,400 seconds. Daylight saving makes one day 23 hours and another 25. Adding n * 86400000 milliseconds drifts by an hour twice a year, which is enough to move a date across midnight and return an answer one day out.

``javascript // Correct — the Date object normalises overflow and handles DST const addDays = (date, days) => { const result = new Date(date); result.setDate(result.getDate() + days); return result; };

// Correct — calendar days, unaffected by DST or time of day const daysBetween = (a, b) => { const utc = (d) => Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()); return Math.round((utc(b) - utc(a)) / 86400000); };`

Months are zero-indexed in JavaScript. new Date(2026, 0, 1) is January. new Date(2026, 12, 1) is January 2027, silently.

Parsing is not portable. new Date("2026-08-30")is parsed as UTC, whilenew Date("2026/08/30")` is parsed as local — the same calendar date, a day apart in some zones. Always parse ISO 8601, and construct with explicit components when the time matters.

Time Zones Decide the Answer

Question	Depends on
How many days until an event?	The viewer's zone
Is this subscription expired?	UTC, always
What day of the week was this?	The zone the event happened in
When does "tomorrow" start?	The viewer's zone

Store timestamps in UTC and convert for display. Storing local times without an offset makes the data unrecoverable — you cannot tell later which zone it meant, and DST transitions make some local times ambiguous or non-existent.

Reference Points

Interval	Days
Week	7
Month (average Gregorian)	30.44
Quarter	91.31
Year	365.2425
Leap year	366

The Gregorian rule — leap year every 4 years, except centuries, except every 400th — keeps the calendar within one day of the solar year for about 3,000 years.

Conception Date Calculator