Representing open datetime ranges using Common Metadata

[duckontheweb]

The start_datetime and end_datetime fields in the Common Metadata spec are required to be currently not allowed to be null, which means that there is not a well-defined way of representing open intervals.

This seems like a reasonable constraint for most EO data, but in the ML Model Extension we want a way to represent the fact that a model could be used on any imagery after a certain date/time. We are currently working around this by recommending that publishers set the end_datetime value to "9999-12-31T23:59:59Z", but using null seems like a more natural way to represent this.

Representing ML models as spatiotemporal assets is admittedly a bit of an edge case given the intended usage of STAC, but I wonder if there would be support for allowing null values for start_datetime and end_datetime to represent these types of open intervals for other use-cases as well.

[m-mohr]

I think that makes sense with the constraint that only one of them should be allowed to be null.

[m-mohr]

Just for reference on nullable geometries: #854