There is a (reasonably) simple way to parse an ISO timestamp WITHOUT the time zone using formulas instead of macros. This is not exactly what the original poster has asked, but I found this question when trying to parse ISO timestamps in Excel and found this solution useful, so I thought I would share it here.
The following formula will parse an ISO timestamp, again WITHOUT the time zone:
=DATEVALUE(MID(A1,1,10))+TIMEVALUE(MID(A1,12,8))
This will produce the date in floating point format, which you can then format as a date using normal Excel formats.