If you have a meeting across multiple time zones and you want to find a suitable time and tell everyone, then you need this. (In case you do not know the time zone you can type their nearest city AND state (for example there are at least two Londons and two Parises), and it will find the time zone for you.)
To find time zones, you can use this map: https://www.randymajors.org/timezonesongooglemaps