An input box with the datePicker popup functionality has the following problem:

When the default configuration of ThickBox and DatePicker are used together, the pop-up calendar will show behind the thickbox panes.

Changing the z-index of div#dp-popup corrects the issue.

div#dp-popup {
position: absolute;
font-size: 10px;
font-family: arial, sans-serif;
background: #ccc;
padding: 2px;
min-height: 172px;
min-width: 171px;
}

to
div#dp-popup {
position: absolute;
font-size: 10px;
font-family: arial, sans-serif;
background: #ccc;
padding: 2px;
min-height: 172px;
min-width: 171px;
z-Index: 199;
}