Linux systems usually synchronise the time with ntp (network time protocol) servers. If the time is not correct, that means either the synchronisation process is not running or is misconfigured.

Linux mint/debian does not use the default ntp deamon, but instead uses ntpdate-debian to update the time. This uses configuration in /etc/default/ntpdate. However, when the full ntp package is installed, this uses the server list of the main configuration in /etc/ntp.conf

sudo ntpdate-debian

To install ntp, just enter this command:

sudo apt-get install ntp

This uses the default ntp servers from the distribution (debian). To add country specific ones, check http://www.pool.ntp.org/ and add them by editing the default ntp configuration:

sudo vi /etc/ntp.conf

Afterwards, restart the ntp service using:

sudo service ntp restart

To check if the ntp service is synchronising, use

sudo ntpq -c lpeer

and finally to check the synchronization log:

grep "ntp" /var/log/syslog