A single date/time, or a date/time range, defining the valid time period that the data pertains to. Dates are specified in the format "YYYY MM DD [HH:MM]". Years in the current time period should be specified using four digits, e.g. "1999" or "2001". Years can have other than 4 digits and can be negative. A date range is specified by supplying two values separated by a "-" character. An optional time can be supplied should this level of accuracy be required. Times are to be specified in 24-hour format with respect to GMT. For example, "1999 01 01 00:00 - 1999 12 31 23:59".