Finding the difference is trivial:
DateTime dt1, dt2;
...
TimeSpan diff = td2 - dt1;
The Timespan struct as several intervals you can use to evaluate the difference:
TimeSpan Structure (System)[
^]
But as F-ES Sitecore says, if the values in your DB are DATE or DATETIME columns, you should just use them directly:
DateTime dtAlert = (DateTime) rows["AlertsendTime"];
DateTime dtSend = (DateTime) rows["DateTimeSend"];
TimeSpan diff = dtSend - dtAlert;
If that doesn't work, it implies that you have a problem in your DB, and the most likely reason is that the columns are VARCHAR or NVARCHAR - in which case the date information is either in the wrong format (MM/DD/YY instead of DD/MM/YY for example) or just plain wrong. So start by using the debugger to look at exactly what the DB is returning, and see what you can find.
We can't do that for you - we don't have access to your data!