When creating or editing a user, you may specify the location(s) from which a user can access your CRM site, such from an office, home, or non-dynamic VPN (where IP address is always the same).
In the Security section of the user page, the "Office Access Only" row(s) will show the IP Addresses from which you may allow a user to sign in, which are in Eastern Time regardless of the user's time zone. To restrict the user to one or more IP Addresses, check the box(es).
To set the IP Addresses, go to Manage >> Users & Groups >> Office IP Security, where you can add, edit, and delete records.
If a user believes they are attempting to sign in from an allowed IP address but are still receiving a "Login not permitted outside the office" error (image below), have them get their IP Address and confirm it matches what is allowed for their user account.