I've got a test page here
showing null start and end times where it should say 'tbc'.
Here's the output syntax:
<% if isnull(rs("ca_starttime").Value) then %>tbc<% else %><%=Right("00" & hour(rs("ca_starttime")), 2)%>:<%=Right("00" & minute(rs("ca_starttime")), 2)%><% end if %>
And here's where it's entered:
<% CreateTimeSelectionBox "starttime", NULL, False %>
If someone could please show me exactly what needs to be written in the output syntax it'd be much appreciated.