Change default time zones for Users and the organization

To change the time zone of the organization

Classic UI

1. Navigate to Setup | Company Profile | Company information | Default Time Zone
2. Select the required time zone.

Lightning Experience UI

1. Click the Gear Icon | Setup | Company Settings | Company Information
2. Change the time zone.

Note: This will be set as the default time Zone for any new Users added to your organization, but the existing Users will continue to have the time zone settings defined for them individually.

 
To change the time zone for the affected users

Note: These steps can be completed by the Users themselves.

Classic UI

1. Go to your name | My Settings | Personal | Language & Time Zone
2. Set the time zone.

Lightning 

1. Click the avatar icon | Settings | My Personal Information | Language & Time Zone
2. Change the time zone.

To change the time zone for the affected users

Note: These steps should be performed by the system administrator.

Classic UI

1. Go to Setup | Manage Users | Users
2. Click on the affected User’s name.
3. Click Edit and change the time zone to the required value.

Lightning UI

1. Click on the gear icon | Setup | Users | Users.
2. Click on the affected User’s name.
3. Click Edit and change the time zone to the required value.

 
If the time zone remains the same for everyone, the values within the date/time fields will remain constant for all Users on the records.

Additional resources:  Select Your Language, Locale, and Currency

Note: The date/datetime fields are fetched in the UTC time zone in the code. So you can use the format method while fetching the details in the code. 
See format method to know more.