you can use this function
Dateadd("n",(<integer>*-1),{@tday})
where n stands for minutes to be added or subtract
(<integer>*-1) is the no of minutes (table field name) to be subtracted
{@tday} is the date from which the minutes to be subtracted.
You can find many more such inbult functions of crystal reports in chapter 6 "Programming with Crystal Syntax" of Brians Bischof book " Crystal Reports Encyclopedia"