It therefore looks as though one nanosecond is going missing somewhere.

Is this intentional -- am I missing something to do with DateTime::Duration objects, or is this down to a bug in one of the DateTime modules?

I inserted this near the failing test

use DDS; diag Dump( $span->duration, $duration );
[download]

and it produced

# $DateTime_Duration1 = {
#                         days     => 0,
#                         end_of_month
#                                  => 'wrap',
#                         minutes  => 0,
#                         months   => 0,
#                         nanoseconds
#                                  => 0,
#                         seconds  => 3600
#                       };
# $DateTime_Duration2 = {
#                         days     => 0,
#                         end_of_month
#                                  => 'wrap',
#                         minutes  => 60,
#                         months   => 0,
#                         nanoseconds
#                                  => 0,
#                         seconds  => 0
#                       };
# bless( $DateTime_Duration1, 'DateTime::Duration' );
# bless( $DateTime_Duration2, 'DateTime::Duration' );
[download]

So it apperas that DateTime::Duration->compare is not normalizing the units so the comparison fails. I consider this a bug.