DateTime input missing - leads to `invalid signature` errors

[joepio]

• Create a Resource with a property of the timestamp datatype (e.g. createdAt)
• Try saving it
• Get an error invlalid signature

I think this may indicate multiple issues:

• The error is confusing, should tell something like datatype mismatch (server side)
• The client should show an error when validating
• We need a dateTime input, e.g. calendar + clock selector