As you may know, Timekpr is an open-source parental control software, allowing admins to choose when a computer can be used and when it cannot. The users’ daily usage can be limited in certain time intervals, accounts can be locked, etc.

The initial timekpr support has been dropped, the latest supported system being Ubuntu 11.10, but Eduards Bezverhijs has adopted the project, added a bunch of extras and made it available for the latest Ubuntu releases.
The latest version available is Timepkr 0.3.5, which has been recently updated.
Installation instructions:
Up to date packages are available via some third party PPA, so installing the software on Ubuntu and derivative systems does not require too much effort. Just add the PPA to your system, update the local repository index and install the timekpr package:
$ sudo add-apt-repository ppa:mjasnik/ppa
$ sudo apt-get update
$ sudo apt-get install timekpr
Optional, to remove timekpr, do:
$ sudo apt-get remove timekpr
The installation instructions should work on Ubuntu 15.10 Wily Werewolf, Ubuntu 15.04 Vivid Vervet, Ubuntu 14.04 Trusty Tahr, Ubuntu 12.04 Precise Pangolin, Linux Mint 17.3 Rosa, Linux Mint 17.2 Rafaela, Linux Mint 17.1 Rebecca, Linux Mint 17 Qiana, Linux Mint 13 Maya, Pinguy OS 14.04, Elementary OS 0.3 Freya, Elementary OS 0.2 Luna, Deepin 2014, Peppermint 6, Peppermint 5, LXLE 14.04 and Linux Lite 2.