Test harness doesn't support Mac OS 10.13

[NightFlyer]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
The test harness on Mac OS 10.13 doesn't correctly deal with date tests because the command line options for date have changed and aren't recognized.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.
Run the tests on Mac OS 10.13

What is the expected behavior?
The test harness works on Mac OS 10.13

Which versions todo.sh are you using?

Doesn't matter. It's the test harness itself that is messing up

Which Operating System are you using?
Mac OS 10.13

Which version of bash are you using?

Run bash --version
Not an issue