Windows with temporal endpoints

[justin13601]

E.g., be able to set the start or end to a specific calendar date 01-01-2023 (May require a new aggregation type beyond temporal (possibly rename to duration) and event-bounded)

Note this is distinct from #187, which states that we should be able to define a constraint based on the timepoints of a window, whereas this is about setting a start or endpoint to occur at a specific timepoint.