"In certain application areas a double hyphen is used as a separator instead of a solidus.". (Notations like 2000--2002 were promoted by previous versions of the standard.)
The ISO 8601 standard does not specify whether a date or time (or date and time) designation refers to a singular point in time or a time period. In particular, a designation of a date can be used to refer to a full 24-hour day or a specific moment of time within it, probably by default the start of the day (00:00). Similarly, a time notation like 9:00 could refer to nine o'clock absolutely sharp or the period from 09:00 to 09:01 or anything else. When necessary, a specific agreement or verbal indication of the meaning can be given, or the most explicit notation with ISO 8601 could be used. For example, one could write 09:00:00 or 09:00:00/09:01:00 to distinguish between the two interpretations mentioned above.
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Also recommend you read the following:
Standard Date and Time Format Strings