Build a list of valid times

validtimes.cs

List<string> times = new List<string>();
            
Enumerable.Range(0, 24).ToList()
    .ForEach(hour => Enumerable.Range(0, 2).ToList()
        .Select(x => string.Format("{0}:{1:00}", hour, x * 30))
        .ToList()
        .ForEach(timeslot=>times.Add(timeslot)));