the DMax() is return the records field max value from a table,not the parameters.
the code what you need is(or you creat a user function):
If parametername = "time" then
rs!Q12005 = iif([Me.Jan_05]> [Me.Feb_05] and [Me.Jan_05]> [Me.Mar-05],[Me.Jan_05],iif([Me.Feb_05] >[Me.Mar-05],[Me.Feb_05]...