Tab-ing out of Calendar ActiveX

[pvp]

Windows 2002 (XP).

I have a Calendar 10.0 ActiveX control in a form. Because
it is too big for the form, I have it permanently
invisible until another placeholder control is reached
(by tab-ing out of another control). This all works fine
except that, to tab out of the Calendar control after I
have used it to enter a date, I have to shift-tab rather
than just tab, which works with all my other controls...

Any suggestions why this is?

Many thanks.