How it decides
The calculator counts completed years between the date of birth and today's date in the UK, using calendar dates rather than dividing days by 365. A person is 18 on their 18th birthday, so someone born on today's date eighteen years ago passes: the switch reads "18 or over" rather than "over 18" so nobody reads it as excluding them. The 21+ block applies the same arithmetic for venues that run a higher age policy.
Today's date comes from the Europe/London time zone whatever the phone is set to, and the page re-checks it every few seconds, so a card left open across midnight updates itself, including both cut-off dates.
29 February birthdays. Under section 9 of the Family Law Reform Act 1969 a person attains an age at the start of the relevant anniversary of their birth. In a year with no 29 February the anniversary is taken as 1 March, which is the accepted reading of the Act: someone born on 29 February 2008 is 17 on 28 February 2026 and 18 on 1 March 2026. The calculator follows that rule.
Worked example
With the UK date at 3 November 2026: a date of birth of 03/11/2008 flips the 18+ switch to Allow with age 18, because today is their birthday, while the 21+ switch stays on Reject (turns 21 on 3 November 2029). A date of birth of 04/11/2008 gives Reject on both, age 17, turning 18 on 4 November 2026, tomorrow. The cut-offs for the day read 3 November 2008 for 18+ and 3 November 2005 for 21+.
Using it on the door
- Keep the full-screen cut-off card open on a phone at the entrance. Anyone whose ID shows a date of birth after the 18+ date is under 18 today.
- Check the ID itself first: photo, hologram, feel, expiry. The calculator only does the arithmetic. Our Door Supervisor course covers acceptable forms of ID and the Challenge 25 process.
- If the date on the ID is impossible, for example 31 April, the ID is wrong, whatever the calculator says.
FAQ
Is someone 18 on their 18th birthday?
Yes. In England and Wales a person reaches an age at the start of the anniversary of their birth date, so they are 18 from midnight on their birthday.
What date of birth is 18 today?
The 18+ block always shows the cut-off: anyone born on or before that date is 18 or over today. The 21+ block shows the same for a 21+ policy.
Does the calculator store dates of birth?
No. The date is checked in your browser and nothing is saved or sent anywhere. Pressing "Check another DOB" clears it.
What is Challenge 25?
A policy where anyone who looks under 25 is asked to prove they are over 18 before buying age-restricted goods or entering an 18+ venue. It is about when to ask; the legal age limit is still 18.