Combo Box Search Query on Null / IsNull / Date Range

[mous]

Hi

I have several combo boxes that lookup values such as (1) Null or Is Null and (2) Date ranges, for example, my second combo box has a list of options to select such as:

Today
Yesterday
Last Week
Last Month
Last Year etc etc

How can I use these values to specify the dates in question.

In terms of the null / is null how can I use the first combo box

Null
Is Null

to select those values held with the query.

Thanks