Your nodes should always have synchronized clocks, if nothing else, because the Chef server requires clients' clocks to be synchronized with it. This is required because the authentication of clients is based on a time window in order to prevent man-in-the-middle attacks.
NTP is there to synchronize your nodes' clocks with its upstream peers. It usually uses a set of trusted upstream peers so that it gets a reliable timing signal.
It's a good idea to put the installation of NTP into a role, which you assign to every node. Bugs caused by clocks, which are out of sync, are not nice to track down. Better avoid them in the first place by using NTP on every node.
Berksfile in your Chef repository including the
mma@laptop:~/chef-repo $ subl Berksfile
mma@laptop:~/chef-repo $ berks install
Resolving cookbook dependencies... Using ntp (1.7.0)
ntp cookbook to the Chef server:
mma@laptop:~/chef-repo $ berks...