deja-dub --backup in a terminal starts backing up immediately.
Second: Open up a terminal and type
crontab -e, press enter and then type
2 to select the nano editor. Enter again and now you have text something like;
# Edit this file to introduce tasks to be run by cron.
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# For more information see the manual pages of crontab(5) and cron(8)
# m h dom mon dow command
So far I understand should following command
0 12 * * * deja-dup --backup written without the
# at start, start the backup at 12 a.m (I suppose a 24h system here).
5 12 * * * deja-dup --backup: Start backup at 12:05 a.m
* 12 5 * * deja-dup --backup: Start backup 5th of every month at 12 a.m
* 6 * 2 * deja-dup --backup: Start backup at 6 a.m in Frebruary
Refer to this askubuntu question to have a better idea about cron jobs in ubuntu