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Theoretically the maximum difference can be 24 hours, but to answer your question, we need to look for time zones that are not defined theoretically but do exist. That is, beyond GMT+12 and beyond GMT–12.
As it happens, beyond GMT–12 does not apply to any time zone. But we do have GMT+13 and GMT+14!
What this means is,
- Time difference between GMT+14 and GMT–12 is that of 26 hours.
- Time difference between GMT+13 and GMT–12 is of 25 hours.
- Time difference between GMT+14 and GMT–11 is of 25 hours.
(Did you note an interesting thing: all the places with these time zones have such huge time differences despite being SO close to each other in the Pacific!)
Regions which use the time zone GMT+14 or GMT+13:
- In 2009, Samoa moved the International Date Line to the other side of its territory, which means that its time zone was changed from GMT–11 to GMT+13. It observes GMT+14 in the southern summer.
- Line Islands of Kiribati observe GMT+14 as standard time.
- Phoenix Islands of Kiribati, Tokelau, and Tonga observe GMT+13 all year round.
- Fiji and New Zealand observe GMT+13 in southern summers.
Regions which use the time zone GMT–12 or GMT–11:
- Baker Island and Howland Island use GMT–12, though they are uninhabited.
- American Samoa and Niue use GMT–11.
So now, from the above sets, pick up your pairs based on the season in the regions at a particular time. The pairs will be such:
- A region with GMT+14 and GMT–12 in a particular season.
- A region with GMT+13 and GMT–12 in a particular season.
- A region with GMT+14 and GMT–11 in a particular season.