Round to the highest whole unit of this field. The value of this field and all fields of a higher magnitude may be incremented in order to achieve this result. The fractional millis that cannot be expressed in whole increments of this field are set to minimum.
For example, a datetime of 2002-11-02T23:34:56.789, rounded to the highest whole hour is 2002-11-03T00:00:00.000.
@param instant the milliseconds from 1970-01-01T00:00:00Z to round
@return rounded milliseconds