Here is what I just tried out:
user@root> show system uptime
Current time: 2011-06-29 07:18:03 UTC
System booted: 2011-06-29 07:09:37 UTC (00:08:26 ago)
Protocols started: 2011-06-29 07:13:46 UTC (00:04:17 ago)
Last configured: 2011-05-06 07:05:19 UTC (7w5d 00:12 ago) by root
7:18AM up 8 mins, 1 user, load averages: 0.09, 0.34, 0.22
user@root> request system reboot at 1106290800
Reboot the system at 1106290800? [yes,no] (no) yes
Shutdown at Wed Jun 29 08:00:00 2011.
user@root> clear system reboot
reboot requested by user at Sun Jun 5 00:00:00 2011
[process id 976]
Terminating...
user@root> request system reboot at 201106290800
Reboot the system at 201106290800? [yes,no] (no) yes
*** System shutdown message from user@root ***
System going down at 08:00
Shutdown at Wed Jun 29 08:00:00 2011.
[pid 989]
user@root> clear system reboot
reboot requested by user at Wed Jun 29 08:00:00 2011
[process id 989]
Terminating...
user@root> request system reboot at 110701
Reboot the system at 110701? [yes,no] (no) yes
error: that time is already past.
user@root> request system reboot at 110107
Reboot the system at 110107? [yes,no] (no) yes
error: that time is already past.
user@root> request system reboot at 110701
Reboot the system at 110701? [yes,no] (no) yes
error: that time is already past.
user@root>
I believe it might have problems with leading 0's when entering this date format.
Can you try it out on your machine? I'm running JUNOS 11.1R2.3