Having the correct time and time zone on your routers and having the time be synchronized across your network is useful to accurately track events that occur on the router and network and to correlate events that occur on different routers. The time and date are used to mark when the files stored on the router, including the active and previous configuration files, the system logging files, and the tracing files, were created or last updated. The messages in JUNOS logging and tracing files are timestamped to mark when events, errors, and problems occurred. All these dates and times are based on the router's time. A number of JUNOS commands, including the ones you use to check the status and uptime of the router, report the current time and the time at which events occurred.
On JUNOS routers, you can configure the time manually by simply setting it. This is generally sufficient for smaller or less complex networks. However, for larger or more global networks, you should use Network Time Protocol ( NTP) to set the time for you. NTP is an IETF standard described in RFC 1305 that synchronizes time across computers and routers on the Internet. The router synchronizes the system time with an NTP server and periodically accesses the server to maintain the correct time. NTP uses a hierarchical system of clock strata to derive time. The top-level stratum 1 clock, also called the primary NTP server, is a computer that is connected to a high-precision accurate clock, such as an atomic clock, or to a radio clock, such as a GPS, Loran, or WWVB, which is the NIST time signal radio station. NTP stratum 2 systems derive their time from a stratum 1 system and are one hop away from a stratum 1 system; NTP stratum 3 systems derive their time from a stratum 2 clock, and are one hop from the stratum 2 machine and two hops from the stratum 1 machine. Note that these hops are NTP hops, not network hops. The systems can be any number of network hops apart. NTP can have up to 16 strata.
You should not confuse the NTP stratum with the telco concept of clocking stratum, which describes frequency accuracy.
The JUNOS implementation of NTP is based on the FreeBSD ntpd utility.
For more information about NTP, see http://www.ntp.org, the web site of the NTP research and development project, as well as the documentation page of the NTP Public Services Project (http://ntp.isc.org/bin/view/Main/DocumentationIndex).
Router Configuration and File Management
Basic Router Security and Access Control
Routing Policy and Firewall Filters