A date selected programatically does not show up on the calendar

I want to produce a calendar with highlighted available dates with the first one being selected automatically when user sees it for the first time. However, calendar.SelectedDate works only partially, i.e. it does select the date but it is not shown as "Selected" on the calendar. I suspect that there must be a problem with date formats. Currently, dates are shown on my computer in the following format: "dd.mm.yyyy". Maybe I have to convert it to a format understandable to SelectedDate?
 
Protected Sub DateSelection_DayRender(ByVal sender As Object, ByVal e As DayRenderEventArgs) Handles DateSelection.DayRender Dim vacationStyle As New Style() With vacationStyle .ForeColor = System.Drawing.Color.Green .Font.Underline = True .Font.Bold = True .Font.Size = 15 End With If ServiceDuration < WeekDayMaxGaps(Weekday(e.Day.Date ...