Check whether this date/time is simultaneous with another. The comparison takes time zones into account: if the two instances have different time zones, they are first converted to UTC before comparing. Note that if either instance is a local clock time (type ClockTime), the result cannot be guaranteed to be correct, since by definition they contain no information about time zones or daylight savings changes. If both instances are date-only, they are considered simultaneous if both their start of day and end of day times are simultaneous with each other. (Both start and end of day times need to be considered in case a daylight saving change occurs during that day.)
