There are two problems there:
1) What do you expect to get as a date time combination from
? Because all you will get is the run time error "Invalid cast from 'Int32' to 'DateTime'."
"1" is not a date.
2) If you subtract two dates, you do not get a date - you get a time interval. "Tomorrow minus last Tuesday" will give you a number of days, not a date. Try using a TimeSpan to receive it instead.
What you are probably trying to do is work out the time interval since the last update:
TimeSpan ts = DateTime.Now - LastUpdateTime;