Is it ensure, insure, or assure?
late 14c., somer tyme, from summer (n.1) + time (n.). Earlier were summertide (mid-13c.), sumeres tid (late Old English). In Britain, as two words, with reference to what in U.S. is daylight saving time, recorded from 1916.
One of the best-known songs of George Gershwin; it comes from the opera Porgy and Bess and begins, “Summertime, and the living is easy&ellipsis4;”