Week Number Calculator

What each input means

The calculator asks for 1 piece of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.

A worked example using the default inputs

With the default inputs, the calculator returns Week 45.

The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.

How this is calculated

ISO 8601: the week containing the first Thursday of the year is week 1, and weeks run Monday to Sunday.

Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.

How dates and times are counted here

Date arithmetic is done on calendar dates, not on a count of milliseconds, so month lengths and leap days behave the way a wall calendar does. That single decision removes almost every off-by-one you see in spreadsheet date maths.

The inclusion convention is the usual source of disagreement. "One week from Monday" is the following Monday, which is seven days later but eight dates if you count both Mondays. Every tool states which convention it used in the result, so two answers that differ by one can both be right.

Daylight saving is not a fixed offset. Zones in the database carry their historical rule changes, which is why a meeting booked across a DST boundary can shift by an hour even though both cities look unchanged.

Three worked examples from this section

1 March to 31 March

Excluding the start date gives 30 days, which is the usual "days between" answer. Counting both ends, as a hotel or a project calendar would, gives 31. Neither is wrong; the convention is the whole difference.

A shift from 14:00 to 22:30 with a 30-minute break

The clock difference is 8 hours 30 minutes. Taking off the unpaid break leaves 8 hours exactly, which is 8.0 in decimal hours for payroll.

Clock difference

8 h 30 m

Less unpaid break

8 h 0 m

Decimal hours for payroll

8.00

09:00 in Sydney, seen from London

During daylight saving Sydney runs at UTC+11. Nine in the morning there is 22:00 UTC the previous day, and London in winter is UTC+0, so the same moment reads as 22:00 on the day before. In July, when Sydney is UTC+10 and London is UTC+1, the gap narrows to nine hours.

How days, months and time zones are counted

Date arithmetic is performed on calendar dates rather than on elapsed milliseconds, so month and year lengths are handled correctly and leap days are respected. Where a difference is reported, the convention used (whether the end date is included) is stated in the result.

• Time zone conversions use the IANA time zone database, which carries each region's daylight saving rules and their historical changes.
• Adding months to the 31st can land on a shorter month; the standard convention is to move to that month's last day.
• Dates are interpreted in your device's local time zone unless the tool asks for a zone explicitly.

Sources and further reading

• IANA time zone database