What does IFormatProvider do?

Passing null as the IFormatProvider is not the correct way to do this. If the user has a custom date/time format on their PC you'll have issues in parsing and converting to string. I've just fixed a bug where somebody had passed null as the IFormatProvider when converting to string.

Instead you should be using CultureInfo.InvariantCulture

Wrong:

string output = theDate.ToString("dd/MM/yy HH:mm:ss.fff", null);

Correct:

string output = theDate.ToString("dd/MM/yy HH:mm:ss.fff", CultureInfo.InvariantCulture);