Rounding Time with NodaTime

There's a sort of functionality that will help: the idea of "adjusters". If you write your own method that returns a Func<LocalTime, LocalTime> you can apply that to LocalTime, LocalDateTime, OffsetDateTime and OffsetTime (in 2.3+) using the With method.

That will help with the truncate case pretty easily:

using System;
using NodaTime;

class Test
{
    static void Main()
    {
        var truncator = CreateTruncator(10);
        var start = new LocalDateTime(2018, 3, 21, 7, 32, 15);
        var truncated = start.With(truncator);
        Console.WriteLine(truncated); // 2018-03-21T07:30:00
    }

    static Func<LocalTime, LocalTime> CreateTruncator(int minuteGranularity) =>
        input => new LocalTime(
            input.Hour,
            input.Minute / minuteGranularity * minuteGranularity);
}

(Note that if you give that a minuteGranularity of 7 or something else that's not a factor of 60, it will still truncate to periods that always start on the hour.)

However, there's no way of that advancing the date. The adjuster mechanism works for a Func<LocalDateTime, LocalDateTime> as well - you can apply that to LocalDateTime, OffsetDateTime. But yes, you'd need to write the code to do that adjustment yourself. As you say, there are lots of different options you might want for rounding, and it's hard for Noda Time to cover all the possible use cases.

I'd definitely recommend writing a method to create an adjuster rather than just writing a method accepting a LocalDateTime and a number of minutes - it provides a way of you passing the adjuster around very easily, and as I've shown, it fits in with Noda Time's own adjustments. (Look in the TimeAdjusters and DateAdjusters classes for the ones we provide out of the box.)

If you need help writing a specific adjuster, that's probably worth asking in a separate question. I wouldn't expect it to need double loops etc.