Here is an app that I've used for years on my desktops, that lends itself quite well to portability on USB drives.
It's the actual NIST Internet Time Service or NIST as commonly referred to. It's a single self executing file (exe), and I'm using it right now on my USB drive to accurately set the time on a PCs internal clock via any internet connection.
It's a handy little tool for use as part of any general tune-up on a friend or customer's PC.
The app can be downloaded from NIST Internet Time Service, at:
http://tf.nist.gov/service/its.htm
Like me, you'll most likely want the version listed as:
"Windows 95 and later (32 or 64-bit)".
Happy App Hunting!
I like Neutron, which performs a similar service. I've not compared the two applications.