When i login to ubuntu, i have no time and date displayed on top-right corner of my ubuntu top bar.

First go to System Settings > Time & Date and check if Show a clock in menu bar is checked. If this is unchecked, just check it, Date and Time will show up.

It if it checked and is grayed (disabled), then it is something with indicator-datetime-service, check if the process is running

boby@fwhlin:~ $ ps aux | grep indicator-datetime-service
boby      3113  0.0  0.1 453148  6616 ?        Ssl  07:15   0:00 /usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service
boby      5257  0.0  0.0  15940   956 pts/26   S+   08:00   0:00 grep --color=auto indicator-datetime-service
boby@fwhlin:~ $

I have the process running with 3113

I done sudo strace -p 3113, it was stuck at futex command.

So i killed the process with command

boby@fwhlin:~ $ kill -9 3113
boby@fwhlin:~ $ 

This auto restarted indicator-datetime-service and Date and Time started showing up.

You can also try running the command from terminal

/usr/lib/x86_64-linux-gnu/indicator-datetime/indicator-datetime-service

This will show the Date and Time, but won’t go in foreground, you can force it with &, but killing existing process will do it.