The second argument in this function call:
dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC)
causes the compiler error:
'UInt64' is not convertible to 'Int64'
Swift Reference:
func dispatch_time(when: dispatch_time_t, delta: Int64) -> dispatch_time_t
var NSEC_PER_SEC: UInt64 { get } /* nanoseconds per second */
See: How do you cast a UInt64 to an Int64? That question doesn't show up when googling the compiler error:
'UInt64' is not convertible to 'Int64'