a <- as.Date(seq(as.POSIXct('1987-05-29 018:07:00'), len=26, by="21 day"))
b <- as.Date(seq(as.POSIXct('2002-05-29 018:07:00'), len=26, by="21 day"))
age <- age_calc(a, units='years')
age
age <- age_calc(a, units='months')
age
age <- age_calc(a, as.Date('2005-09-01'))
age
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