Not sure what exactly your issue is about. Suppose if you are here in India, your user timezone will be Asia/Kolkata , ie, +5:30 UTC. And if you save a date-time value 26/06/2019 6:30 PM in your custom field, in the database it will get stored in the UTC as 26/06/2019 1:00 PM(by subtracting the +5:30 from the time).
And when the same field has to be displayed in the front end from the database, the UTC time from the database is taken and displayed according to the users timezone. So when displaying the same field to the user with the timezone as Asia/Kolkata it will be 26/06/2019 6:30 PM, and for another user with timezone Saudi Arabia which is +3 , the value will be 26/06/2019 1:00 pm + 3 hours, ie 26/06/2019 4:00 PM.
In the database 26/06/2019 1:00 PM,
for user 1 with timezone +5:30 , time will be 26/06/2019 06:30 pm.
for user2 with timezone +3:00, time will be 26/06/2019 04:00 pm.