Require DateTimes to be in UTC (...Z)

[kaestli]

In the current XSD, times are just defined as xsd:datetime.
While in many local stationXML documents, fdsn/station responses, no time zone is given and seismologists may implicitly assume that times are in UTC, the XMLSchema standard actually says on the data type xsd:datetime:
"If no time zone value is present, it is considered unknown; it is not assumed to be UTC." - times with unknown timezones are not very useful for the user.
Moreover: The character strings allowed in xsd:datatimes (e.g. 2004-04-12T13:20:00) look like ISO 8601-formatted time strings. However, ISO 8601 says: "If no UTC relation information is given with a time representation, the time is assumed to be in local time." - if a client interprets such a time without time zone e.g. using the standard java implementation of ISO 8601 time parsing, (s)he will get a time object with the time zone according to the locale of his machine.

To avoid these ambiguities I would suggest that all times should be given in UTC explicitly, and hence all time stamps have a respective zone mark ("...Z")

[chad-earthscope]

I agree, the time zone should be explicit.

@kaestli By what mechanism would it be enforced in the schema?

[crotwell]

see #12 discussion and for possible schema change to do this

see also #54 for documentation issue