If it's not in the AP Stylebook, dictionary or here, it's two words. as a noun or Verb, hyphenated as adjective. A able-bodied .. nonsensical no one. 0 oceangoing oddsmaker off-Broadway off-color off-duty offhand . summertime sundress.
The word summertime (like springtime) is a compound word, and the two - word form summer time may mean something completely different -- when capitalized.
Spellweb is your one -stop resource for definitions, synonyms and correct spelling for English words, such as summertime. On this page you can see how to spell. How to Improvise over Summertime using chords