If you want to convert a UTC time to a local time (and I assume with local time in your example you mean North Dakota) you have to create the time in UTC and then change to local timezone. I tried it with your script with switched time zones and it seemed to work as expected.

Now DateTime documentation explicitely says it doesn't parse data. Since you have fixed date strings that are easy to parse I would suggest using a simple regex to parse your dates and use DateTime->new() with that data

PS: Are you sure Guernsey has always UTC time, with or without Daylight Saving Time?