I managed to solve this problem by adding
z-index: 1000;
in skin css file.

for example, my modified calendar-win2k-cold-2.css now starts with:

.calendar {
position: relative;
display: none;
border-top: 2px solid #fff;
border-right: 2px solid #000;
border-bottom: 2px solid #000;
border-left: 2px solid #fff;
font-size: 11px;
color: #000;
cursor: default;
background: #c8d4d0;
font-family: tahoma,verdana,sans-serif;
z-index: 1000;
}