However you need to consider that in this sense, it's about what is happening at any given moment in time. In your case, the forecasts are created at a point in time, and that "should" be their index speaking purely from a time-is-the-ultimate-index perspective.
Just some random questions that might help you answer your own questions: 1. Are there multiple forecasts made at the same time, for the same period in the future, but by different systems / algorithms / hyperparameters? Would you not want to keep multiple, and in that regard, what would be the "latest" forecast? 2. If your latest forecast is necessarily the most accurate, why would you need to keep previous versions in the same database?
None of this might be relevant to you, and maybe you'll only get the benefits of InfluxDB by using the forecast time as the index. But I thought I'd give you my thoughts just in case it helps you.
You can express forecasts this way. The valid time range is the range for which you assert a given forecast is going to be true. The transaction time range is the time during which you held that forecast to be the current forecast.
Using transaction time you can then reconstruct your state of belief from any point in time. Using valid time you can make assertions about any fact in ranges over the past, present or future.
I think Snodgrass's Developing Time-Oriented Database Applications in SQL[0] is still the best overall introduction, though the SQL used is fairly dated (ha). The relevant Wikipedia entry is OK too[1].
