Field #3 (Formula field resulting in a number): Figure out the offset in hours between our time zone specified in field #2 (when applied to field #1) and the GMT time zone specified in field #1.
Field #4 (Formula field resulting in Date/Time set to GMT): Airtable still thinks that the GMT time specified in field #1 is the actual time of our event, but it’s not. So we still need to figure out the REAL GMT TIME of the event in field #1, which is based on the number of offset hours that we calculated in field #3. Then, this field #4 will have the ACTUAL GMT TIME OF THE EVENT. This is the actual GMT time of the event. This field can be hidden from the users, because they won’t need to see this.
Field #5 (Formula field resulting in Date/Time with GMT turned off): Same formula as #4, but since GMT is turned off, every user logging into the system will see this field updated with their local time.