It looks like you could change your code to return an IEnumerable<DateTime>
pretty easily.
//Get your distinct time spans
var distinctTimeSpans = workingHours.Distinct();
//Build date objects from the parameter and time span objects
var dates = distinctTimeSpans.Select(ts => new DateTime(date.Value.Year, date.Value.Month, date.Value.Day, ts.Hours, ts.Minutes, ts.Seconds));
Then you can call ToString()
on your DateTime
object: .ToString("hh:mm tt")