*! version 1.0.0  20nov1996 STB-34 sts12
program define _crcchkt
	version 5.0
	local if	"opt"
	local in	"opt"
	local options	"t(string)"
	parse "`*'"

	xt_tis `t'

	local realt "$S_1"
	tempvar tt
	gen `tt' = `realt'-`realt'[_n-1] `if' `in'
	summ `tt' `if' `in'
	capture assert _result(5)==_result(6) `if' `in'
	if _rc {
		noi di in red "time variable does not have constant step size"
		exit 198
	}
	if _result(5) == 0 {
		noi di in red "tied values in time variable not allowed"
		exit 198
	}
end
	

exit

Here is how we will use this:

program define <progname>
	version 5.0
	local varlist 	"req ..."
	local if	"opt"
	local in	"opt"
	local options	"t(string) ..."
	parse "`*'"

	_crcchkt `if' `in', t(`t')

	...
end