According to this blog post, the key factor is whether a treaty relinquishing control of the British Indian Ocean Territory is signed.

Resolution Criteria

Resolves YES if, in 2025 (GMT), there is a moment when it is nighttime (i.e. between sunset and sunrise) for all territories under the control of the United Kingdom. Resolves NO if this never happens.

General policy for my markets: In the rare event of a conflict between my resolution criteria and the agreed-upon common-sense spirit of the market, I may resolve it according to the market's spirit or N/A, probably after discussion.