Currently, the ADL grammar does not require (or allow) the use of the 'T'
character in Date/time strings, however it is mandated by the
ISO 8601 standard.
Also, in the date/time and time constraint patterns it
would be clearer to include the 'T' for the sake of consistency.
Lastly, patterns like 12:— : are currently used to represent partial
times and dates, but ISO 8601 does not allow the use of '?' - this should