The header does not render properly when using the datetime picker in a scollable page.

[dimsavva]

Consider removing position: relative; in the following css

.ui.calendar .ui.table tr:first-child th {
position: relative;
padding-left: 0;
padding-right: 0;
}

[lubber-de]

The calendar module was merged into https://github.com/fomantic/Fomantic-UI and is further maintained there.
However, i cannot reproduce your issue, could you please provide a jsfiddle for easier investigation and being able to reproduce?
What exactly is meant by "not render properly" ?