.NET DateTime, different resolution when converting to and from OADate?

I think this is an excellent question. (I just discovered it.)

Unless you're operating with dates quite close to the year 1900, a DateTime will have a higher precision than an OA date. But for some obscure reason, the authors of the DateTime struct just love to truncate to the nearest whole millisecond when they convert between DateTime and something else. Needless to say, doing this throws away a lot of precision without good reason.

Here's a work-around:

static readonly DateTime oaEpoch = new DateTime(1899, 12, 30);

public static DateTime FromOADatePrecise(double d)
{
  if (!(d >= 0))
    throw new ArgumentOutOfRangeException(); // NaN or negative d not supported

  return oaEpoch + TimeSpan.FromTicks(Convert.ToInt64(d * TimeSpan.TicksPerDay));
}

public static double ToOADatePrecise(this DateTime dt)
{
  if (dt < oaEpoch)
    throw new ArgumentOutOfRangeException();

  return Convert.ToDouble((dt - oaEpoch).Ticks) / TimeSpan.TicksPerDay;
}

Now, let's consider (from your question) the DateTime given by:

var ourDT = new DateTime(634202170964319073);
// .ToSting("O") gives 2010-09-16T06:58:16.4319073

The precision of any DateTime is 0.1 µs.

Near the date and time we're considering, the precision of an OA date is:

Math.Pow(2.0, -37.0) days, or circa 0.6286 µs

We conclude that in this region a DateTime is more precise than an OA date by (just over) a factor six.

Let's convert ourDT to double using my extension method above

double ourOADate = ourDT.ToOADatePrecise();
// .ToString("G") gives 40437.2904679619
// .ToString("R") gives 40437.290467961888

Now, if you convert ourOADate back to a DateTime using the static FromOADatePrecise method above, you get

2010-09-16T06:58:16.4319072 (written with "O" format)

Comparing with the original, we see that the loss of precision is in this case 0.1 µs. We expect the loss of precision to be within ±0.4 µs since this interval has length 0.8 µs which is comparable to the 0.6286 µs mentioned earlier.

If we go the other way, starting with a double representing an OA date not too close to the year 1900, and first use FromOADatePrecise, and then ToOADatePrecise, then we get back to a double, and because the precision of the intermediate DateTime is superior to that of an OA date, we expect a perfect round-trip in this case. If, on the other hand, you use the BCL methods FromOADate and ToOADate in the same order, it is extremely improbable to get a good round-trip (unless the double we started with has a very special form).

The static method called by ToOADate clearly divides the ticks by 10000 and then stores the result in a long, thus removing any sub millisecond info

Does anyone know where to find the specs of the OADate format?

    private static double TicksToOADate(long value)
    {
        if (value == 0L)
        {
            return 0.0;
        }
        if (value < 0xc92a69c000L)
        {
            value += 0x85103c0cb83c000L;
        }
        if (value < 0x6efdddaec64000L)
        {
            throw new OverflowException(Environment.GetResourceString("Arg_OleAutDateInvalid"));
        }
        long num = (value - 0x85103c0cb83c000L) / 0x2710L;
        if (num < 0L)
        {
            long num2 = num % 0x5265c00L;
            if (num2 != 0L)
            {
                num -= (0x5265c00L + num2) * 2L;
            }
        }
        return (((double)num) / 86400000.0);
    }