If you
buy Sun Gold tomato seeds, the planting times are as follows:
"For greenhouse cultivation: sow from February to March.
For outdoor cultivation: sow from March to April."
Source:
http://www.thompson-morgan.com/vegetables/vegetable-seeds/tomato-seeds/tomato-sungold-f1/840TM?source=google&gclid=CLeDirPztLkCFaKWtAodbXsA_Q
However planting seeds taken from existing plants simply won't produce the results that you're hoping for because the plants are F1 hybrids.
Quote:
" . . .seed collected from an Fl hybrid will not produce plants the same as those from which it is collected. Only by crossing the pure lines can the variety be made - and only the original breeder has the necessary pure lines"
Source:
http://www.thompson-morgan.com/f1-hybrid-what-is-it