When you sort in Ex2010, a cell with a value of "cleared/null" (set so
via mouse menu "Clear Contents" or vba range.ClearContents.) gets you
a result with nulls last in both sorts - ascending and descending.

However, if you set the cell's "empty value" as an empty string, = "",
they are considered "values" and sort. I see posts that claim "" ==
null. It doesn't seem so.

I have a formula that, based on existence of a date, sets a value in a
cell. I want either a date or CLEAR/NULL but can't find something
that actually clears or nulls the value. I'm guessing I'll have to
create a UDF (if I can actually set a null with a return "value"). Is
that correct?

Have

=IF(
AND(
NOT(ISNA(VLOOKUP(D10,nt_po_number,2,TRUE)))
),
VLOOKUP(D10,nt_po_number,2,TRUE),
""
)

Want

=IF(
AND(
NOT(ISNA(VLOOKUP(D10,nt_po_number,2,TRUE)))
),
VLOOKUP(D10,nt_po_number,2,TRUE),
NULL
)