https://www.newtonsoft.com/json/help/html/T_Newtonsoft_Json_Converters_IsoDateTimeConverter.htmNewtonsoft JSON has a built-in date & time converter class, IsoDateTimeConverter, using a time format string of "yyyy-MM-ddTHH:mm:ssZ". However, usage of the converter is discretionary. And, it's possible to derive new converter classes from it that set a different format string in their c'tor method.
What you have is still ISO 8601 compliant, even though the precision is truncated to whole seconds instead of going out to some decimal fractional amount. If the trailing "Z" is missing, then the time is assumed to have an unknown time zone. If the "Z" has been replaced with a
"+" or "-" followed by "HH:mm", then it's a local time in a time zone represented by an offset from GMT.
What is going to be key when dealing with dates in JSON is that there simply is no one single unified representation of a date & time value as a string for purposes of interchange via JSON. As a result of this, there has to be either a contract where the provider & consumer agree on a format, or the consumer simply has to be aware of all possible representations and must be capable of handling them all. You might be able to do some pattern matching with a RegEx that would permit some kind of heuristic for automatically choosing a date & time "decoder", but it won't be inexpensive in terms of performance and it won't be 100% reliable.
caveat emptor, YMMV, and all other suitably applicable disclaimers, etc...