JSFiddle - React, Tailwind, and code Playground

HTML

<div class="span12" style="padding: 10px;">
    <form action="<?php echo base_url()?>index.php/chamadas" method="get">
        <div class="span2">
            <label for="">Data Inicial:</label>
            <input type="date" name="dataInicial" class="span12" />
        </div>
        <div class="span2">
            <label for="">Data Final:</label>
            <input type="date" name="dataFinal" class="span12" />
        </div>
        <div class="span2">
            <label for="">&nbsp;</label>
            <button class="btn btn-inverse span12"><i class="icon-print icon-white"></i> Filtrar</button>
        </div>
    </form>
</div>

JavaScript

var inicial = document.querySelector('[name="dataInicial"]');
var final = document.querySelector('[name="dataFinal"]');
inicial.addEventListener('change', function () {
    final.value = this.value;
});