Date duration calculator
- Runs in your browser
- No signup
- Formula shown below
- Reviewed
A date duration calculator measures the period between two dates in days, weeks and months. The duration from 1 January 2026 to 25 December 2026 is 358 days, which is 51 weeks and 1 day, or 11 months and 24 days. That period contains 256 weekdays and 102 weekend days.
Result
Days between the two dates
358 days
days = (end − start) ÷ 86,400,000 ms, computed in UTC
- Years, months, days
- 0 y 11 m 24 d
- Whole weeks
- 51 w 1 d
- Fortnights
- 25 f 8 d
- Weekdays
- 256
- Weekend days
- 102
- Total hours
- 8,592
The duration between January 1, 2026 and December 25, 2026 is 358 days, or 51 weeks and 1 day. That period contains 256 weekdays and 102 weekend days, and equals 11 months and 24 days.
How to use the date duration calculator
- 01
Enter the start date
Type or pick the earlier date of the range. The order does not matter — the calculator returns the same duration whichever way round the two dates are entered.
- 02
Enter the end date
Type or pick the later date. Use the day after the final day if you need an inclusive count of both endpoints.
- 03
Read the duration
The result strip shows the period in calendar days. The rows beneath convert the same interval into years, months, whole weeks, fortnights and hours.
- 04
Check the working-day split
The weekday and weekend rows separate the span for project planning, notice periods and contractual deadlines.
The formula
days = (end − start) ÷ 86,400,000 ms, computed in UTC
- start
- The earlier calendar date.
- end
- The later calendar date.
- days
- Whole calendar days between the two, counting the start day as day zero.
- 86,400,000
- Milliseconds in a day: 24 × 60 × 60 × 1,000.
Weekday counting uses whole-week arithmetic — every complete seven-day block contributes exactly five weekdays — then examines only the remainder day by day. Computing in UTC avoids daylight-saving transitions, which would otherwise make two days in the year 23 or 25 hours long and corrupt a naive division.
Worked example
- Start date
- 1 January 2026
- End date
- 25 December 2026
- Result
- 358 days
The interval spans 358 calendar days. Dividing by seven gives 51 complete weeks with a remainder of 1 day. Each complete week contributes exactly 5 weekdays, giving 255, and the single remaining day is examined individually and is a weekday, bringing the total to 256 weekdays and 102 weekend days. Expressed as a calendar period the interval is 11 months and 24 days, and as hours it is 8,592.
Frequently asked questions
How do you calculate the duration between two dates?
Subtract the earlier date from the later one and divide by the number of milliseconds in a day. The result counts the start day as day zero, so 1 January to 2 January is one day. Counting both endpoints — an inclusive count — adds one, which is the convention used for hotel nights and hire periods.
What is the difference between calendar days and business days?
Calendar days count every day including weekends and public holidays. Business days count only Monday to Friday, and formal definitions usually also exclude public holidays. Contracts, court deadlines and shipping estimates frequently specify business days, so a 10-business-day period spans at least 14 calendar days.
Why does adding months give a different answer than adding days?
Months have unequal lengths, so month arithmetic is not fixed-length arithmetic. Three months from 31 January lands on 30 April, since no 31 April exists — but 90 days from 31 January lands on 1 May in a common year. Legal periods expressed in months follow the calendar convention, not the day count.
How many working days are there in a year?
A common year contains 261 or 262 weekdays depending on which day of the week it begins; a leap year contains 262 or 263. Subtracting public holidays gives the practical figure: about 250 in the United States with 11 federal holidays, and about 253 in the United Kingdom with 8 bank holidays in England and Wales.
Does this calculator account for daylight saving time?
Daylight saving has no effect on the result. All arithmetic runs in Coordinated Universal Time, which has no daylight-saving transitions. Calculating in local time would make two days a year 23 and 25 hours long, and a naive division by 24 hours would then produce off-by-one errors across any span containing a clock change.
How do you convert a date duration into weeks or fortnights?
Divide the day count by seven for weeks, or by fourteen for fortnights. A 358-day period is 51.14 weeks — 51 whole weeks and 1 day — and 25.57 fortnights, meaning 25 complete fourteen-day blocks with 8 days spare. Fortnightly payroll cycles count only the complete blocks. The weeks between dates calculator leads with these figures rather than the day count.
How many weekends fall within a date range?
A weekend counts when both its Saturday and its Sunday sit inside the range. The period from 1 January 2026 to 25 December 2026 contains 51 complete weekends alongside 102 individual weekend days. The two figures differ whenever a range starts or ends mid-weekend, so a lone trailing Saturday adds a weekend day without adding a whole weekend.
How do you calculate a deadline a set number of days away?
Add the period to the start date, then check whether the result falls on a weekend or public holiday. Most legal and administrative rules roll a deadline landing on a non-working day forward to the next working day. Where the rule specifies business days, count only Monday to Friday from the day after the triggering event.
Sources
- ISO 8601 — date and time representation — International Organization for Standardization
- Federal holidays — US Office of Personnel Management
- Coordinated Universal Time and civil timekeeping — US National Institute of Standards and Technology
Last reviewed: · Formula and sources verified by Syed Aqeel Ahmad Gillani. See the methodology for how every calculation is derived.