Custom Fields- Support for pOS datetime field type

For a client project I need to be able to store a historical date in a custom field in a module which cannot be stored as an integer.e.g. 1900 - the integer as a timestamp overflows the length of an integer field.

Please add minimal support for the pos date field for custom fields, so that

1) setting a field to datetime in CLI does not break the admin

2) The admin will not overwrite this field

We don't require full Admin support for this field type at this time.