for date fields, derive day resolution from multiple sources

[dogversioning]

Users should not need to worry about the semantic difference between things like date/instance/period - we should set a priority order and walk down it until we find a valid value, and keep them from having to hunt and peck around for it.

[mikix]

set a priority order

Thankfully, we don't need to stress about that specific detail, since there can be only one format variant provided.

But yes, I like this idea. Good value-add for core

[dogversioning]

relates to #101