Well, it's not the most efficient code, but maybe ...
<font face=Tahoma New><SPAN style="color:#00007F">Option</SPAN> <SPAN style="color:#00007F">Explicit</SPAN>
<SPAN style="color:#00007F">Sub</SPAN> BoldAllDates()
<SPAN style="color:#00007F">Dim</SPAN> cel <SPAN style="color:#00007F">As</SPAN> Range, rng <SPAN style="color:#00007F">As</SPAN> Range, i <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Long</SPAN>
<SPAN style="color:#00007F">For</SPAN> i = 1 <SPAN style="color:#00007F">To</SPAN> Sheets.Count <SPAN style="color:#00007F">Step</SPAN> 1
<SPAN style="color:#00007F">Set</SPAN> rng = <SPAN style="color:#00007F">Nothing</SPAN>
<SPAN style="color:#00007F">Set</SPAN> rng = Sheets(i).UsedRange
<SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> cel <SPAN style="color:#00007F">In</SPAN> rng
<SPAN style="color:#00007F">If</SPAN> IsDate(cel) <SPAN style="color:#00007F">Then</SPAN>
cel.Font.Bold = <SPAN style="color:#00007F">True</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">Next</SPAN> cel
<SPAN style="color:#00007F">Next</SPAN> i
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
</FONT>
And if you had a tremendous amount of data, I would not suggest it. With minimal data, it should not take very long to run at all though.