Here is what I discovered yesterday, and spent couple of hours trying to fix it.
Let's say we want to assign dnn calendar to textbox.

<asp:TextBox ID="txtPaymentDate" runat="server" Width="300"></asp:TextBox>
<asp:HyperLink ID="HyperLink1" runat="server" ImageUrl="~/images/calendar.png">Calendar</asp:HyperLink>

Code behind will be following:

HyperLink1.NavigateUrl = DotNetNuke.Common.Utilities.Calendar.InvokePopupCal(txtPaymentDate);

So, using hyperlink we can invoke popup calendar. Pretty easy and straightforward.

But, approach produces an IE7 crash, if our website supports multiple languages.
For example, when I invoke popup call in en-US locale, everything works fine.
BUT, when I do the same in i.e. ka-GE locale, IE7 just crashes.

I don't know why this happens but the workaround is to use asp:Image instead of Hyperlink, and add OnClick attribute to invoke calendar.