A helper function to switch from "new" t values to original t values. This allows for periods not being exactly spaced apart by 1.
Examples
original_time.periods <- c(2001, 2003, 2005, 2007)
t2orig(1:4, original_time.periods) ## returns c(2001, 2003, 2005, 2007)
#> [1] 2001 2003 2005 2007
