formatSubmit gives blank output on POST

I am using Materializecss' version of pickadate.js. I can't seem to get any output when I set formatSubmit. Here are my settings for pickadate below:

`$('.datepicker').pickadate({
formatSubmit:'yyyy-mm-dd',
hiddenName:true,
selectMonths: true,
selectYears: 100,
container: 'body',
max:true

});`

Here is my HTML inside my form:

`
`

When I hit the submit button I get output for the other fields but birthdate just returns blank. Am I doing something wrong? Any help is appreciated, thanks.