How to convert a UTC DateTimeOffset to a DateTime that uses the systems timezone

You can just use the DateTimeOffset.LocalDateTime property:

trigger.GetNextFireTimeUtc().Value.LocalDateTime

From the documentation:

If necessary, the LocalDateTime property converts the current DateTimeOffset object's date and time to the local system's date and time. The conversion is a two-step operation:

• The property converts the current DateTimeOffset object's time to Coordinated Universal Time (UTC).
• The property then converts UTC to local time.

You should really look into DateTimeOffset though - it's an important type to understand if you're using the BCL for date/time work.