mocha: undefined is not a function --- > myDate.getTimezoneOffset()

I'm running a test suite in Mocha, and getting the above error within my function pointing to native JS methods like getTime() or getTimezoneOffset()

Can someone point me in the right direction here, not sure why this would throw errors.

TypeError: 'undefined' is not a function (evaluating 'myDate.getTime()')

The setup is JavaScript using require.js, running the tests in Node.