How to convert UTC Date Time to Local Date time without TimeZoneInfo class?

i know Tehran is a GMT offset of +03:30

Well, that's its offset from UTC in standard time, but it's currently observing daylight saving time (details). So the current UTC offset is actually +04:30, hence the difference of an hour.

I suspect you're really off by more than an hour though, are you're adding an offset of 3.3 hours, which is 3 hours and 18 minutes. The literal 3.30 doesn't mean "3 hours and 30 minutes", it means 3.30 as a double literal. If you want 3 hours and 30 minutes, that's 3 and a half hours, so you'd need to use 3.5 instead. The time in Tehran when you posted was 9:46 PM... so I suspect you actually ran the code at 9:44 PM.

This sort of thing is why you should really, really, really use a proper time-zone-aware system rather than trying to code it yourself. Personally I wouldn't use TimeZoneInfo - I'd use my Noda Time library which allows you to either use the Windows time zones via TimeZoneInfo, or the IANA time zone database. The latter - also known as Olsen, or TZDB, or zoneinfo, is the most commonly-used time zone database on non-Windows platforms.