Efficient and accurate age calculation (in years, months, or weeks) in R given birth date and an arbitrary date

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The reason lubridate appears to be making mistakes above is that you are calculating duration (the exact amount of time that occurs between two instants, where 1 year = 31536000s), rather than periods (the change in clock time that occurs between two instants).

To get the change in clock time (in years, months, days, etc) you need to use 

as.period(interval(start = birthdate, end = givendate))

which gives the following output

 "37y 0m 1d 0H 0M 0S"   
 "37y 0m 0d 0H 0M 0S"   
 "36y 11m 30d 0H 0M 0S" 
 ...
 "46y 11m 30d 1H 0M 0S" 
 "47y 0m 0d 1H 0M 0S"   
 "47y 0m 1d 1H 0M 0S"

To just extract years, you can use the following

as.period(interval(start = birthdate, end = givendate))$year
 [1] 37 37 36 53 53 52 50 50 49  1  1  0 46 47 47

Note sadly appears even slower than the methods above!

> mbm
Unit: microseconds
       expr       min        lq       mean    median         uq        max neval cld
 arithmetic   116.595   138.149   181.7547   184.335   196.8565   5556.306  1000  a 
  lubridate 16807.683 17406.255 20388.1410 18053.274 21378.8875 157965.935  1000   b

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