Handle TimePeriodType Endtime being a relative value

[DerAndereAndi]

• If a duration is provided for a TimePeriodType EndTime value, and no StartTime is provided in a write message, the endTime should be converted to an absolute timestamp
• A helper method should provide the current duration being left
• On serialization the current remaining duration should be used
• On every subsequent write, the same process should be repeated.