Received thru the DSS-X public comments list from Neil Crossley on the 2019-08-23:

The JSON schema and Swagger schema are consistent in their modelling of date-time:
Swagger:
https://swagger.io/docs/specification/data-models/data-types/#string
JSON schema draft 4 validation:
https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-7.3.1
JSON schema latest validation:
https://json-schema.org/latest/json-schema-validation.html#rfc.section.7.3.1

That is, time is represented as a string with a specific format. The JSON schema declaration looks like this:
type: 'string'
format: 'date-time'

Instead, dss-core consistently employs the following:
type: 'number'
format: 'utc-millisec'

The use of number instead of string does not conform to the standard. While a lossless transformation is possible, I would like to point out that this is a custom, non-standard declaration that may not always be supported by the employable tools.