You will need to substitute your own NTP peers, timezones, and GMT
offsets into the examples below, of course.  Example 1 is in US Central
Time Zone, while example 3 is in US Pacific Time Zone.  Both account
for normal US Daylight Savings Time practices.

EXAMPLE 1 (Charley Kline):
...
clock timezone CST -6
clock summer-time CDT recurring
ntp source eth 0
ntp peer <host1>
ntp peer <host2>
ntp peer <host3>
...


EXAMPLE 2 (Tony Li):
...
ntp source Ethernet0/0
ntp update-calendar
ntp peer <host1> 
ntp peer <host2> prefer
...


EXAMPLE 3 (Dave Katz):
...
service timestamps debug datetime localtime
service timestamps log datetime localtime
clock timezone PST -8
clock summer-time PDT recurring
interface Ethernet0
ip address <mumble>
ntp broadcast
ntp clock-period 17180319
ntp source Ethernet0
ntp server <host1>
ntp server <host2>
ntp server <host3>

COMMENTS ON EXAMPLE 3: 
        The config file is commented with date and time (and user id,
if TACACS is enabled) when the system thinks the clock is accurate.
I've enabled timestamping of debug and syslog messages.  I send NTP
broadcast packets out onto the local ethernet.  I'm in Pacific
Standard Time, with U.S. standard daylight saving time rules.  I use
the IP address of the ethernet as the source for all NTP packets.