Getting your system date from an Ubuntu terminal
First, to get your current Date, use the date command
date +%Y%m%d
For example, printing the Date:
ubuntu@codetryout:~$ date +%Y%m%d
20210105
Setting the system date from commandline
Now, Setting the date and time using the -s switch
date +%Y%m%d -s 20201201
Testing
Let us test it:
ubuntu@codetryout:~$ date
Tuesday 05 January 2021 07:00:05 AM IST
ubuntu@codetryout:~$ sudo date +%Y%m%d -s 20201201
20201201
ubuntu@codetryout:~$ date
Tuesday 01 December 2020 12:00:03 AM IST
Note: This is tested on Ubuntu 20.04. Date command is common across all Linux distributions, will work on Any Linux / Debian distros.