[UK-CONTEST] what's the time?

[G4FNL]

Folks

I was recently challenged on air, because I had started a contest some 20 or
so seconds early. Initially, I was a bit put-out at being questioned - as I
thought my clock was correct, and so I did argue the point. Anyway, on
hearing the call of the challenger - I took note, stopped and started to
wonder whether I was at fault?  Subsequently, I found out that I was!  So,
firstly, I am grateful for the comments. Secondly, I wanted to do something
to prevent this recurring. So, I got wondering how this had happend?

I use a standard Windows XP laptop PC. It is used daily at work. It is
normally connected to a network (and the Internet). The clock is
synchronised to one of the registred time sources on the Internet (in my
case time.windows.com). During contests, it is normally operating
stand-alone

When I checked against the 'speaking clock', my PC was approx 20 or so
seconds fast.

I started looking at time sources on the Internet, and found that there
appears to be quite a discrepancy between sources of 'accurate' time. (I do
understand that there is latency on the Internet - but I wouldn't have
expected such large discrepancies in time (several tens of seconds))

So, my question: has anyone found a reliable time source that I can point my
PC towards that will provide reasonably accurate time synchronisation to
help me start (and finsh) the contest on time?

73 Graham G4FNL