rose date: fix offset and print_format usage

It was failing when both were used together.
Merge branch 'master' of github.com:metomi/rose into gh-pages
Merge branch 'gh-pages' of github.com:metomi/rose into gh-pages
Merge pull request #798 from matthewrmshin/fix-rose-date-offset-print_format

rose date: fix offset and print_format usage
Merge branch 'master' of github.com:metomi/rose into gh-pages