• The Birthdate Calculator – online calculator, of use to genealogists and family history researchers for detailed data for family trees.

Blog posts all about it:

• The Birthdate Tool is born – why a birthdate tool is needed
• Why isn’t birthdate arithmetic easy? Birth Years. – how birth year is calculated
• Birthdates and time zones – and why they don’t matter
• Birthdates and Leap Years. – and why they do matter
• Birthdates and the Month. – Oh, Julius, why did you have to invent months?
• Birthdate: time arithmetic is not reversible! – time from now back to birthdate is not the same as time from birthdate up to now
• Birthdate: UK Census calculator – working from published census returns
• Birthdate: birth registration index quarter – working from published birth indexes
• Birthdates when the calendar changed – which are rather weird, and not handled yet
• Birthdates: how to save options – so you don’t have to type them in again
• Birthdate Custom Readouts – so you can copy results from the birthdate calculator straight into your family tree program
• Birthdate at Github – if you want to change the code