If you want to subtract 7 hours from the date as your input string says, use RelativeDate function as below:
=FormatDate(RelativeDate(ToDate(left(Trim([Input String]);19);"yyyy-MM-dd HH:mm:ss");-7/24);"yyyy-MM-dd HH:mm:ss")
If the number of hours to be added/subtracted varies, then you need to separate that part from the input string using substr function and form the factor to be added or subtracted in relativedate function.