DateTimeZone.switchZone using zone ID not time offset

So my problem is I want to get my NZT  date time to a UTC datetimeZone so I can calculate duration difference from DateTimeZone.UtcNow()  

NZT is utc +12 untill last Sunday in September at 02:00  when it becomes +13 for the period of daylight saving. 

I cannot just use DateTimeZone.ToUtc([ColumnName])  because I only have a DateTime with no Zone.

I first need convert the NZT datetime to UTC by subtracting the offset duration.

[DateTimeColumnName]- #duration(0,12,0,0)    

Then DateTime.AddZone([dateTime], 0) to make it as UTC DateTimeZone  

Now I can calculate the duration difference with DateTimeZone.UtcNow()   

When I need to report the actual date I use DateTimeZone.Switch([UTCDateTimeZone, 12)   to get it to NZT format.

The problem is that the time Offset  changes from +12 to +13 on September 26 2021  at 02:00

I want to be able  to do the following DateTimeZone.Switch([UTCDateTimeZone, NZT)  rather than DateTimeZone.Switch([UTCDateTimeZone,13)

My work around is to create a table with Daylight Saving Periods as below

I then look up the offset using the dateTime I have and use the Offset Value in  DateTimeZone.Switch([UTCDateTimeZone, [OffSet])

But this is a time consuming workaround it woult be much easier to just use the timezone id in switch eg  DateTimeZone.Switch([UTCDateTimeZone, NZT). I have logged this as an Idea.