AngularJS, how do I combine multiple inputs into a single ng-model? Specifically, to combine a date, time, and timezone input into a datetime object

I need to create a datetime object to store in MySQL, but I need it to be entered with multiple inputs (one for date, one for time, one for timezone). So right now my code is like this:

<input type="datetime-local", ng-model="workshop_date">

But what I want is something more like this:

 <input type="date", ng-model="date">
 <input type="time", ng-model="time">
 <select ng-model="zone">
     <option>Pacific</option>
     <option>Eastern</option>

 <input value='{{date + time + zone}}', ng-model="workshop_date", style="display:none">

However, this solution seems to confuse Angular. Is there any way I can take multiple inputs and make that the value of the ng-model?