Incorrect clientID for Calendar control as part of web user contro

[MarcDeVos.BE]

I have a web user control which contains a textbox (ID TextBox1) and a calendar (ID Calendar1).

When I put several of them on a Web Form (let's say 2 and give them IDs Test1 and Test2) and run the page, in the generated HTML, the IDs are :
.. For the Test1 textbox : <input id="Test1_TextBox1" ...> which is correct
.. For the Test1 calendar : <table id="Calendar1" ...> which is not correct
.. For the Test2 textbox : <input id="Test2_TextBox1" ...> which is correct
.. For the Test2 calendar : <table id="Calendar1" ...> which is not correct

So I have no way in referencing the correct calendar from client-side script.