help with MS Access query.

There is a point at which a 'compressed' code goes from elegant to confusing, and I believe you've crossed it my friend haha. I'm all for elegance, but sometimes readability has to factor into the equation. I'm not too familiar with VBA or Access VBA, I only know a bit, so I'll just kinda psuedocode it for you.

If IsDate(LastChkDt)
    If IsDate(Last_PO_Date)
        If LastChkDt > Last_PO_Date
            'Use LastChkDt
        Else
            'Use Last_PO_Date
        End If
    Else
        'Use today's date
    End If
Else If IsDate(Last_PO_Date)
    'Use Last_PO_Date
Else
    'Use today's date
End If
' There are plenty of ways to shorten (and potentially optomize) this code, but this is the best way I can devise without knowing more about VBA, using redudant if conditions, or spending more time than I have at the moment. The code isn't pretty, so definitely take the time to beautify it if possible :)

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Are both date fields in the same format?

Access doesn't allow "else if".  the only option is: IIF(comparison, true, false)

which means i have to use nested IIF.

and i think i'm missing a parenthesis or something.

yes, both date fields are the same format.

if i do:
IIf(IsDate([LastChkDt])=0,IIf(IsDate([Last_PO_Date])=0,0,1),1)

i get a 0 when both of the fields are empty.  i get a 1 when at least one of them is a date.  i've also separated the test out into two separate columns to verify isdate() is correctly detecting when each of the two fields have a date, or when they are empty.

rockiroads - this solution is working...
except when both fields are null, i'm getting a date/time result for today's date.

i tried changing now() to date(), but i'm still getting date/time.

Try using the format command to show just the mm/dd/yyyy.

how?

i tried:
format(IIf(IsNull([dbo_qwcs_MPVEND]![LastChkDt]) And IsNull([dbo_qwcs_MPVEND]![Last_PO_Date]),date(),IIf(NZ([dbo_qwcs_MPVEND]![LastChkDt],CDate("01/01/1900"))>NZ([dbo_qwcs_MPVEND]![Last_PO_Date],CDate("01/01/1900")),[dbo_qwcs_MPVEND]![LastChkDt],[dbo_qwcs_MPVEND]![Last_PO_Date]),"mm/dd/yyyy")

and i'm still getting date/time

WTF?
i didn't change anything, and now it's not showing the time.  just the date.

IIf(IsNull([dbo_qwcs_MPVEND]![LastChkDt]) And IsNull([dbo_qwcs_MPVEND]![Last_PO_Date]),Date(),IIf(NZ([dbo_qwcs_MPVEND]![LastChkDt],CDate("01/01/1900"))>NZ([dbo_qwcs_MPVEND]![Last_PO_Date],CDate("01/01/1900")),[dbo_qwcs_MPVEND]![LastChkDt],[dbo_qwcs_MPVEND]![Last_PO_Date]))

That works for me in access 2010, once I added the missing parenthesis before the last comma.

format(IIf(IsNull([dbo_qwcs_MPVEND]![LastChkDt]) And IsNull([dbo_qwcs_MPVEND]![Last_PO_Date]),date(),IIf(NZ([dbo_qwcs_MPVEND]![LastChkDt],CDate("01/01/1900"))>NZ([dbo_qwcs_MPVEND]![Last_PO_Date],CDate("01/01/1900")),[dbo_qwcs_MPVEND]![LastChkDt],[dbo_qwcs_MPVEND]![Last_PO_Date])),"mm/dd/yyyy")

Sorry, I had to pop out so couldn't answer your query. I always tend to use Now() as Date() sometimes gives reference issues. Now() does give the time as you have noticed.

You have it working now anyways but an alternative to date() is DateValue(Now())