HTTP Time Protocol - FAQ

Why should I use htpdate?
You shouldn't, use ntp if you can. Ntp is much more accurate.... however ntp doesn't work in combination with proxy servers. Also ntp traffic (UDP port 123) is often blocked by firewalls and doesn't always work if you are behind a NAT device. If you have access to websites, htpdate will work.

How accurate is htpdate?
It depends on the accuracy of the time stamps provided by the choosen web servers. With multiple servers the accuracy increases. In general htpdate will keep your time at least at timesource±0.5 seconds. Take a look at here to get an impression.

Which web servers can I use for time synchronization?
All web servers are required to return a time stamp in their response. The quality/accuracy isn't always, what you might expect. Use htpdate with the "-d" switch, to find some good web servers close to you.

How many server should I use with htpdate?
"A man with two watches, can never tell the right time". Three servers is the recommended minimum. Just in case one server is way off, I would recommend to use 4 servers. A maximum of 16 web servers are supported by htpdate.

Which algorithme does htpdate use?
Read the source... (for the moment)

How much bandwidth does htpdate use?
Very, very little (too little to worry about, but if you do, read the source).

Does htpdate support leap seconds?
No.
At the and of December 2005, a leap second will be added. Htpdate will adjust the second offset, as if a second offset was detected.

Are there any other alternatives for time synchronization?
Yes, rdate and timed.