Year month string to date
Usage
ym_diff(ym = c(200902, 201912), ym2 = c(200901, 201712))
References
http://stackoverflow.com/questions/1995933/number-of-months-between-two-dates/26640698#26640698
Examples
ym_diff(c(200902, 201912), c(200901, 201712))
#> [1] 1 24