If a child was born today, which is July 18th and the third Monday of July, would he or she become one month old on August 18th or the third Monday in August (August 22)?


  • 1
    Months are not a specific unit of measurement, so I'd say it's up to you.
    – DA01
    Jul 18, 2011 at 21:25
  • If my baby was born on August 3, 2015 how old is she now..
    – user20104
    Dec 8, 2015 at 19:42
  • Well, his birthday being on the 18th, i would say it's the same each and every month. Same way you don't ask yourself if he's 1 year old after 365 days or at the same date the next year...
    – Laurent S.
    Dec 14, 2015 at 14:37
  • What if baby is born on the 31st of a month?
    – user20307
    Dec 26, 2015 at 5:51

6 Answers 6


This is a trick question. Each month does not consist of a whole (integer) number of weeks, except February, or equal number of days.

Just choose what you like best and stick with that. I would choose the n'th of the month and say my birthday is on the 18th, because that's the easiest for everybody.

The only exception is if your birthday is February 29th. Then you need to know that the 29th is not actually the leap day, the 25th is! So in non-leap years your birthday is Feb 28th, not March 1st. If your birthday is on the actual leap day (25th) then I guess the 26th will do as birthday in non-leap years.

Counting weeks is only relevant for infants, when you see the pediatrician. He'll know anyway, so don't bother about that. Once the child is 8 weeks old, switch to counting months. Once he's 24 months old, switch to counting (half) years.

  • "Then you need to know that the 29th is not actually the leap day, the 25th is!" ?? Can you provide some more info on that? I find it especially interesting, because my mom's birthday is the 25th.
    – Shauna
    Jul 18, 2011 at 20:03
  • @Shauna, just now I googled to find a source and didn't even find any details in Wikipedia... but it does say that my knowledge seems to be out of date. Jul 18, 2011 at 20:52
  • Possibly because you were off by a day: en.wikipedia.org/wiki/February_24 and more importantly: en.wikipedia.org/wiki/Julian_calendar Mind you, back then they added an entire 13th month, that day just came and went periodically.
    – cabbey
    Jul 18, 2011 at 22:02
  • 2
    The leap day is pushed in between 23 and 24, so the leap day will be 24 and the previous 24 is now 25, the previous 25 is now 26, and so on. This is purely calendar-academic blah-blah though, in practice it's totally irrelevant. Jul 19, 2011 at 8:52
  • 1
    +1 for total calendar geekery. I now have another piece of nigh-useless lore with which to impress my friends who will no doubt exclaim "Why the #$@! do you know that?!"
    – afrazier
    Jul 19, 2011 at 20:36

I calculate month birthdays like I calculate year birthdays: By date.

Ie, the 29th of April, our daughter was one year and one month old. As she is born the 29th, this means she wouldn't have an actual birthday most Februaries, but so what? It's not like you have a party for monthly birthdays anyway. It's just a matter of how many months you say she is, and for that you usually switch a couple of days before anyway. Nobody expects you to say that your kid is "One year, 5 months and 28 days old". You say "18 months".

But if you want to be exact: http://howoldismykid.com/


We always did it by day of the month, since months don't all have the same number of days we went from date of month to date of month. So in your example a baby born on July 18 would be one month old on August 18.

  • 1
    Us, too. But it doesn't really matter. Jul 18, 2011 at 16:11

Being an engineer who works on things like recurring payments, I tend to over-engineer the answer for my wife's liking. A month it turns out is really really close to 30.5 days. So I tend to use that. Which means each month is either 30 or 31 days... alternating. (note that this is much easier to use when using a date calculator that will let you just say "2007-08-07 + (30.5 * 18)".)


The only real world numbering system is days from start of year, where that point is arbitrarily chosen as 10 days after winter solstice (typically in the western world)

Weeks are an affectation defined to work with the way humans traditionally work and rest. Months are also just traditional, and have changed over time - see the Julian and Gregorian calendars for example.

However the generally accepted practice is that you use the day of the month - eg born on 18 June, 1month old on 18 July etc.


I do it by day of the month not by number of weeks. 4 weeks doesn't make a month. We have 52 weeks in a year. If we calculate by 4 weeks a month then we need to have 13 months in a year. Just calculate this way. So your baby will be one month old on August 18th.

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