Zapier - once every two weeks interval from certain date

every-two-weeks.js

// 2020, 2, 16 === march 16th, 2020
let startDate = new Date(2020, 2, 16);
let nowDate = new Date(inputData.year, parseInt(inputData.month) - 1, inputData.day)
let milisecondsPerDay = 24 * 60 * 60 * 1000;

// Check if it's in a 14 days interval from now.
if (Math.round((nowDate - startDate) / milisecondsPerDay) % 14 === 0) {
    return { "continue": true }
}

return { "continue": false }

Set up in Zapier:

Then go and filter based on the "continue" key: