$ sudo apt-get update
$ sudo apt-get install indicator-datetime-gtk2 

indicator-datetime-gtk2 is installed in your system.

Make ensure the indicator-datetime-gtk2 package were installed using the commands given below,

$ sudo dpkg-query -l | grep indicator-datetime-gtk2 *

You will get with indicator-datetime-gtk2 package name, version, architecture and description in a table

indicator-datetime-gtk2 package basic information: