nested z-index doesn't work - css

Question

I have a series of divs with z-index:20 (this is a must be for visual styling reasons), but inside one of them, I have a datepicker which stands as position:absolute floating above everything. I've assigned z-index: 1000 as a great value in order to achieve this.
I discovered the inner z-index this doesn't work because of the parent container. Is there such a hack to workaround this ?
The parent container must have such z-index in order to be above a sibling div, but the datepicker, which is inside one of the boxes, must be on top of everything, and now it is hiding under the next box.
This is the codepen with an actual example: http://codepen.io/anon/pen/jWZMPw

Answer 1

The problem is that .box creates a stacking context because it's a positioned element with non-auto z-index.
Don't do that. Remove
.box {
z-index: 20;
}
.banner {
padding: 10px;
background: #454545;
position: fixed;
width: 100%;
}
.box {
position: relative;
float: left;
margin-left: 20px;
min-width: 100px;
min-height: 100px;
border: 1px solid #999;
padding: 20px;
background: #45e;
}
.box:first-child {
margin-left: 0;
}
.dtpicker {
background-color: #34ed22;
padding: 20px;
min-width: 100px;
min-height: 100px;
position: absolute;
left: 50px;
z-index: 50;
}
<div class="banner">
Something behind
</div>
<div class="box" >
Box
</div>
<div class="box" >
Box
</div>
<div class="box" >
Box
<div class="dtpicker">
Dt picker
</div>
</div>
<div class="box" >
Box
</div>
I recommend reading What No One Told You About Z-Index.