Changing the time zone on your computer does change the meaning of DateTimeZone.local. It does not change the underlying DateTimeData value used to represent the actual date and time, since that is always stored in UTC time; it only determines how to convert the DateTimeData to the DateTimeInfo, which breaks the time down into minutes and seconds and so on.

The native OS's Curl supports only provide timezone information back to 1970 (Windows might do better than that but we are reluctant to introduce many OS-specific behavior in our runtime libraries) so before that we only provide UTC offset timezones.

So if you really need to represent a time before 1970, you should initialize it using a UTC timezone. Then you should create a function to produce the UTC version of the local timezone as I outlined earlier, and use that function to set the timezone of the DateTime whenever you think the computer's timezone might have changed (unfortunately, Curl does not provide an event to to tell you when this happens).

Here is how I think I might write the timezone conversion function:

{define-proc public {local-timezone-for-date date:DateTime}:DateTimeZone
   || Find year within 1970-2038 that is closest to 'date' since that is mostly likely
   || to have similar timezone characteristics.
   let reference-year:int
   {if date.info.year <1970 then
       set reference-year = 1972 || first leap-year after 1970
    elseif date.info.year >= 2028 then
       set reference-year = 2036 || last leap year before 2038
    else
        {return DateTimeZone.local}
   }
   def reference-date =
       {DateTime 
            year = reference-year,
           month = date.info.month, 
           day = date.info.day
        }
   {return
       {DateTimeZone
           mode = "utc",
           utc-offset-minutes = reference-date.info.utc-offset-minutes
        }
    }
}