We use NTP to synchronise all our servers. I believe that the EC2 time inside instances is actually sourced from the host machine (there's some Xen setting for this which escapes me at the moment).AWS Developer Forums: How accurate are EC2 date/time clocks? ...
# date 05251435 Mon May 25 14:35:00 JST 2009 # date Mon May 25 14:31:07 JST 2009
# cat /proc/sys/xen/independent_wallclock 0