Google has some common types they use for their APIs, but they're also commonly used elsewhere:
https://github.com/googleapis/api-common-protos/

Among them are some useful types like DateTime, Date, TimeOfDay & DayOfWeek.
I was wondering whether these types could be added to prost-types or a separate crate.

In the .NET ecosystem both Well-Known Types & Google Common Protos are supported. Google.Protobuf contains the main Protobuf implementation as well as the Well-Known-Types under Google.Protobuf.WellKnownTypes while Google.Api.CommonProtos adds support for Google Common Protos.

On a slightly different note:
Maybe prost-types could optionally support conversion to chrono types for Timestamp & Duration.
This would also be useful for new types from Google Common Protos.