The calendar rules and where they come from
Each rule our calculators use, with its source and the date we last checked it.
| Rule | Figure | Source | Checked |
|---|---|---|---|
| A leap year comes every this many years | 4 years | Calendar (New Style) Act 1750, section 2 | 14 November 2026 |
| Hundredth years are common years, not leap years | 100 years | Calendar (New Style) Act 1750, section 2 | 14 November 2026 |
| Except every fourth hundredth year, which is a leap year | 400 years | Calendar (New Style) Act 1750, section 2 | 14 November 2026 |
| Days in a common year | 365 days | Calendar (New Style) Act 1750, section 2 | 14 November 2026 |
| Days in a leap year | 366 days | Calendar (New Style) Act 1750, section 2 | 14 November 2026 |
| British Summer Time starts on the last Sunday of this month | 3 month | Summer Time Order 2002 (amending section 1 of the Summer Time Act 1972) | 14 November 2026 |
| British Summer Time ends on the last Sunday of this month | 10 month | Summer Time Order 2002 (amending section 1 of the Summer Time Act 1972) | 14 November 2026 |
| The hour, in Greenwich mean time, at which summer time starts and ends | 1 o'clock GMT | Summer Time Order 2002 (amending section 1 of the Summer Time Act 1972) | 14 November 2026 |
| Seconds in a minute | 60 seconds | NIST Guide to the SI, section 5.1.1, Table 6: units accepted for use with the SI | 14 November 2026 |
| Minutes in an hour | 60 minutes | NIST Guide to the SI, section 5.1.1, Table 6: units accepted for use with the SI | 14 November 2026 |
| Hours in a day | 24 hours | NIST Guide to the SI, section 5.1.1, Table 6: units accepted for use with the SI | 14 November 2026 |
| Mothering Sunday is the Fourth Sunday of Lent, three Sundays before Easter Day | 3 weeks before Easter Day | Church of England, Common Worship: The Calendar and the Date of Easter and other variable dates | 14 November 2026 |
| All Saints' Day, whose eve (All Hallows' Eve) is Halloween | 1 November | Church of England, Common Worship: The Calendar and the Date of Easter and other variable dates | 14 November 2026 |
Bank holidays
Easter and Christmas bank holidays come from GOV.UK: UK bank holidays, using its data feed, checked 13 November 2026.
Week numbers
Week numbers follow the international standard ISO 8601: weeks start on a Monday, and week 1 is the week with the year's first Thursday in it.
Today's date
Calculators that count from today use today's date in the UK, from your device's clock, so they change at midnight UK time.