What is the most reliable World Clock Webservice that you use?
Since there are so few web services out there delivering time, http://www.timeapi.org/utc/now is only reliable web service that I know of (besides http://www.earthtools.org/timezone/0/0, which does not appear to be reliable). Therefore it's the most accurate one I can recommend, especially if you are just using it for determining the difference between local time and UTC time, which can be rounded to the nearest 15 minutes. And if you want the time in a specific time zone, replace utc
with the three-letter abbreviation for the time zone -- i.e., http://www.timeapi.org/est/now for the Eastern Standard Time.