Various flavors of TD are of great importance in astronomical calculations. Based on this system, noon at 1 January 2000 was JD 2451545.0. For astronomers, this system offers several advantages. If you want to know the number of days elapsed between two different dates, you can just subtract the JD of one instant from that of another. You'll very rarely deal with dates before the year -4712, so the JD value will almost always be positive.
Time zones are no longer a worry.
For Daylight Savings time, subtract one hour less than this. This causes some troubles, because the Earth's rotation is somewhat irregular due to tidal effects from the moon. This sort of Universal Time is called UT1. Those days can therefore be 86401 seconds long instead of 86400 seconds long. Since the earth's rotation is irregular, the insertion of leap seconds is also irregular.