We have a geography tag. That includes physical geography, which is on-topic, but also questions more related to what people do with the Earth, such as What is the line crossing the Pacific Ocean that is shown on maps?. Is the latter on-topic on Earth Science?
It seems to me that the International Date Line is a human construct used to delineate an arbitrary border for the purpose of measurement of time on the Earth.
Therefore, by definition, any question pertaining to the International Date Line in the context of the question is an Earth Science question.
They could sometimes be off-topic, but there is not a geography stack and we we have the tag. They are on-topic in my opinion most of times as they are related with geoforms (geomorphology) or with oceanography and geomorphologists and oceanographers can develop a very complete answer (most of cases more complete than a geographer would do).
I would only close questions about pure geography as demography, not related with the forms of Earth's surface.
It is not the case of the questions I have seen that have always some relation with natural sciences and not with human sciences; then on-topic I guess.
I agree that asking topics that are clearly about human geography, or about human constructs relating to physical geography (e.g. the international date line) should perhaps be off-topic here. The guiding principle might be "is this about the earth, or about people?". But we need to be careful, and not close things too readily, and for this reason I'd be reluctant to make this a "rule" - because on Stackexchange, whenever there is a written rule, some people will immediately vote to close if they can possibly apply that rule ;-) Off the top of my head I'd urge restraint in two categories, but there are probably others:
- Things that are about geography, but are also about earth science. For example, geography and geology or geomorphology can have heavy crossovers (arguably one is the cause of the other), and should be on-topic here. This will be a large proportion of geography questions.
- Things that are only about (human) geography, but reasonably appear to be about earth science to somebody who does not already know the answer. Yes, I'm thinking about this question: Let's not close things that are only off-topic if you already know the answer
This question would not be on topic at https://gis.stackexchange.com/ as seen by the tag info here - https://gis.stackexchange.com/questions/tagged/geography. So I do not think it fits anywhere else. Maybe OP can talk to the GIS mods and make them see our point.