Use this parseDateTime('1/1/' + toString($TimeDimension/Year), 'MM/dd/yyyy')
You can use the parseDateTime function
You have to first make the $DateString from your other values.
parseDateTime($DateString, '<the format in which you have your DateString>')
For time zone adjustment you can use any of these functions: parseDateTime OR parseDateTimeUTC
Please refer to documentation for details: https://docs.mendix.com/refguide/parse-and-format-date-function-calls
Here, you can do it like this.