Use Java's ZonedDateTime. Ignition's datetime objects are java.util.Date which are fundamentally UTC under the hood, with no attached time zone. Which means they always use the time zone of the JVM when parsing or printing. When you manipulate them with .addHours(), you are not making a time zone correction, but are altering the underlying UTC value. Don't use .addHours() to make time zone corrections.
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Note also that DateTimeFormatter can parse, too.