Is there a simple function for rounding a DateTime down to the nearest 30 minutes, in C#?

Question

For example:

2011-08-11 16:59 becomes 2011-08-11 16:30

Answer 1

A more generic solution rounding to the nearest time span:

public static DateTime RoundUp(this DateTime dt, TimeSpan d)
{
    var delta = (d.Ticks - (dt.Ticks % d.Ticks)) % d.Ticks;
    return new DateTime(dt.Ticks + delta);
}

public static DateTime RoundDown(this DateTime dt, TimeSpan d)
{
    var delta = dt.Ticks % d.Ticks;
    return new DateTime(dt.Ticks - delta);
}

public static DateTime RoundToNearest(this DateTime dt, TimeSpan d)
{
    var delta = dt.Ticks % d.Ticks;
    bool roundUp = delta > d.Ticks / 2;

    return roundUp ? dt.RoundUp(d) : dt.RoundDown(d);
}

It would be used this way:

var date = new DateTime(2010, 02, 05, 10, 35, 25, 450); // 2010/02/05 10:35:25
var rounded = date.RoundToNearest(TimeSpan.FromMinutes(30)); // 2010/02/05 10:30:00

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