Mac OS X Date & Time Synchronization

NTPd (used by Mac OSX) will sync at the speed it thinks is more appropriate - that depends on your clock skew. It will be more frequent when you first turn it on / connect to the net and then will connect at longer intervals when it has stabilized your clock enough.

It does not only sync your current time and date, but also sync the actual clock speed. That means that after some time that you have enabled it (and are connected to the internet) your clock will be very precise even if you stay disconnected from the 'net for long times.

If you want the gory details, head on to wikipedia and to the NTP site.

Update

Since OS X 10.14 (Mojave) ntpdate command has been removed - use sntp instead:

sudo sntp -sS time.apple.com

Original answer

For older versions of OX X (pre 10.14), the original answer below is still applicable.

To manually update time with NTP, use ntpdate from a shell:

ntpdate -vu (server or IP address)

e.g.,

sudo ntpdate -vu time.apple.com

References:

• Archived copy of ntpdate(8) (for “Mac OS X version 10.9”)
• More current (but still obsolete) copy
• ntpd - Network Time Protocol (NTP) daemon
• Sntp User’s Manual

ntpdate options:

-v

Be verbose.  This option will cause ntpdate’s version identification string to be logged.

-u

Direct ntpdate to use an unprivileged port for outgoing packets.  This is most useful when behind a firewall that blocks incoming traffic to privileged ports, and you want to synchronise with hosts beyond the firewall.  Note that the -d option always uses unprivileged ports.

-q

Query only – don't set the clock.

Disclaimer: The functionality of this program is now available in the ntpd program. 
See the -q command line option in the ntpd - Network Time Protocol (NTP) daemon page. 
After a suitable period of mourning, the ntpdate program is to be retired from this distribution.

The network time daemon (ntpd) logs activity to /var/log/system.log. You can see recent activity with grep:

$ grep ntpd /var/log/system.log | grep "time reset"
Jun 18 22:28:56 mymac ntpd[28]: time reset -0.301952 s
Jun 19 10:37:26 mymac ntpd[28]: time reset -1.443661 s
Jun 20 09:46:07 mymac ntpd[28]: time reset -3.529638 s
Jun 21 09:57:43 mymac ntpd[28]: time reset -3.293325 s
Jun 21 22:25:11 mymac ntpd[28]: time reset -0.539650 s
Jun 21 22:43:12 mymac ntpd[28]: time reset +0.142553 s
Jun 22 09:24:23 mymac ntpd[28]: time reset -1.844003 s
Jun 22 09:41:59 mymac ntpd[28]: time reset -0.156320 s
Jun 23 09:06:00 mymac ntpd[28]: time reset -1.880272 s

Read the ntp.conf man page (man ntp.conf) for details on further configuring ntpd including how much info is logged and (I assume) frequency of updates.