Because archives are “non destructive”, i.e. it doesn’t actually change any of the data, it’s perfectly acceptable to have overlapping seasons.
e.g. 2011 Summer = 2011-04-01 – 2011-08-31
2011 Winter = 2011-08-01 to 2012-05-31
Archives can be edited and deleted at anytime without any effect on your data.
If you need any help adding the archives, email firstname.lastname@example.org.