Support date in the spec?

[ThomasG77]

In https://github.com/mapbox/tilejson-spec/blob/master/3.0.0/example/osm.json, it could be interesting to set a date as optional.

Why this request?

Knowing when the vector tiles have been updated.

Limitations:

It will work better with vector tiles served from static source like an mbtile.

Otherwise, it's not always relevant if tilejson possible date is not synced with vector tiles due to dynamic vector tiles endpoint.

Maybe It's already solved if considering I optional key management as it seems from https://github.com/mapbox/tilejson-spec/blob/master/3.0.0/example/osm.json#L17 and excerpt from the spec below

Implementations MUST treat unknown keys as if they weren't present. However, implementations MUST expose unknown key value pairs so users can optionally handle these keys. Implementations MUST treat invalid values for keys as if they weren’t present.