[timed] Use nanotime_t operators when calculating time change

The operators in nanotime_t do various adjustments to the
values stored internally, whereas the getters like
nanotime_t::sec() do not.

For example, comparing nanotime_t::sec() with an integer value
will produce unpredictable results.
Merge pull request #41 from pgerdt/use-nanotime_t-operators

[timed] Use nanotime_t operators when calculating time change