In order to allow for unforeseen time zone database adjustments, Junos can be configured to use a custom time zone database by editing an existing time zone database file. This feature implies that non POSIX-compliant time zone string can be accepted, when using the 'set system time-zone' command. Using the nonexistent time zone file name, the candidate configuration can be committed; but, by default, the time zone is set to UTC.

The set system time-zone command is committed in any time zone the string is inserted in. No error or failed message is displayed and, by default, the time zone is set to the UTC time zone.

root@vegas-re1# set system time-zone ?
Possible completions:
<time-zone> Time zone name or POSIX-compliant time zone string

root@vegas-re1# set system time-zone jtac
root@vegas-re1# commit
commit complete

 From Junos documentation:

'The time zone database information on JUNOS routers can be updated independently of the JUNOS operating system. This feature simplifies time zone management in JUNOS systems by allowing for future unforeseen time zone database adjustments. You can configure your router to use a custom time zone database file that you create to meet your requirements by editing an existing time zone database file'.

When using the standard time zone values, users should be sure of the spelling. Otherwise, the time zone is set to UTC.

When using  a customized time zone database file, user should ensure that:

• use-imported-time-zones is configured under system.

• There is an existing time-zone file corresponding to the non POSIX compliant string.

• Setting a Custom Time Zone on Routers Running JUNOS Software
• KB9892 POSIX-compliant time-zone string [juniper.net]