Round Down to nearest boundary in dateTime

Question

In this answer of a similar question the DateTime is rounded to the closes (time) boundary, the Math.Round method does not allow round to the lower boundary by choice.
Is there a way to calculate the same way the lower boundary of some time ?
Meaning if the time is 10/2/2012 10:52:30 and the choice is an hour than the time is : 10/2/2012 10:00:00, if the choice is day than 10/2/2012 00:00:00 and so on.

Are you planning to use your `DateTime` value in arithmetic later on or just display it rounded? — M.Babcock, Jan 08 '12 at 08:36
Then you likely need a string anyway. Why not just use a variation of @JonSkeet's answer or just `DateTime.ToString()`? — M.Babcock, Jan 08 '12 at 08:41

Jon Skeet · Accepted Answer · 2015-06-25T05:27:06.063

If you only need to go to a particular unit, I probably wouldn't even bother using Math.Round or Math.Floor - I'd just go with something like:

switch (unitToRoundDownTo)
{
    case Unit.Second:
        return new DateTime(old.Year, old.Month, old.Day,
                            old.Hour, old.Minute, old.Second, old.Kind);
    case Unit.Minute:
        return new DateTime(old.Year, old.Month, old.Day,
                            old.Hour, old.Minute, 0, old.Kind);
    case Unit.Hour:
        return new DateTime(old.Year, old.Month, old.Day, old.Hour, 0, 0, old.Kind);
    case Unit.Day:
        return new DateTime(old.Year, old.Month, old.Day, 0, 0, 0, old.Kind);
    case Unit.Month:
        return new DateTime(old.Year, old.Month, 1, 0, 0, 0, old.Kind);
    case Unit.Year:
        return new DateTime(old.Year, 1, 1, 0, 0, 0, old.Kind);
    default:
        throw new ArgumentOutOfRangeException();
}

That doesn't work if you need "the nearest 5 minutes" etc, but for a single time unit it's simpler to understand and debug than trying to get arithmetic to work.

Alternatively, as a different spin on the accepted answer to the question you link to, you can just do:

// tickCount is the rounding interval, e.g. TimeSpan.FromMinutes(5).Ticks
DateTime rounded = new DateTime((old.Ticks / tickCount) * tickCount);

Note that this won't help for rounding to the start of the month or year.

It's probably a good idea to maintain the kind as well, which you can accomplish by adding `, old.Kind` as the final parameter to the DateTime constructor. — Brad, Jun 24 '15 at 22:17
@Brad: Yes, that's true. Will edit. (Not that I'm keen on the kind part, but...) — Jon Skeet, Jun 25 '15 at 05:25

score 3 · Answer 2 · answered Jan 08 '12 at 08:32

3

Try Math.Floor instead of Math.Round (in a similar fashion to the post you linked).

answered Jan 08 '12 at 08:32

Eve Freeman

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Round Down to nearest boundary in dateTime

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