It is doable, but first of all you need to consider how you will parse the marked dates to Datetimepicker.
Should the dates be parsed as one or more parameters in NewCssCal(..) and if so how many parameters (=dates) will you include knowing that the dates must cover from the beginning of time to forever? Or should the calendar be able to obtain the dates for the current month using xmlHttpRequest()?
Either way it requires some nerdy rewriting of the calendar.