Convert any date math string to milliseconds. Check Elastic's documentation for further reading (Note: rounding and conditionals are not supported yet).
npm install datemath-to-milliseconds --save
var convert = require('datemath-to-milliseconds');
console.log(convert('now-15s'));
// => -15000
console.log(convert('now+5m'));
// => 300000
console.log(convert('now-7h'));
// => -25200000
console.log(convert('now+4d'));
// => 345600000
console.log(convert('now-3w'));
// => -1814400000
console.log(convert('now+2M'));
// => 5184000000
console.log(convert('now-1y'));
// => -31536000000
console.log(convert('now-15s+5m-7h+4d-3w+2M-1y'));
// => -27845715000