returns the difference between 2 dates
num=diff_date(lastdate,firstdate)
* lastdate = last date
* firstdate = first date
* num = the number of observations between the 2 dates
d = diff_date('1995a','1985a') // gives 10 d = diff_date('1987q4','1985q2') // gives 10 too d = diff_date('72m1','83m12') // gives -143