I love this standard. If you dig deeper into it, the standard also covers a way to express intervals and periods. E.g. "P1Y2M10DT2H30M" represents one year, 2 months, 10 days, 2 hours and 30 mins.
I recall once using the standard when writing a cron-style scheduler.
I also like the POSIX "seconds since 1970" standard, but I feel that should only be used in RAM when performing operations (time differences in timers etc.). It irks me when it's used for serialising to text/JSON/XML/CSV.
Also: Does Excel recognise a full ISO8601 timestamp yet?