Not sure how that query returns anything because syntax is not correct. Should be:
SELECT Count(*) AS NumberOfRows FROM (SELECT DISTINCT InvDt FROM tblTodaysIds) AS T;
Build and save a query object like:
SELECT DISTINCT InvDt FROM tblTodaysIds;
This will create a dataset of all unique dates found in tblTodaysIds.
Now in VBA use DCount().
"Here are today's Ids." & vbCrLf & IIf(DCount("*","QueryName")>1, "MORE THAN ONE DATE IN TODAY'S FILE","")
Or use your original nested query and DLookup()
"Here are today's Ids." & vbCrLf & IIf(DLookup("NumberOfRows","QueryName")>1, "MORE THAN ONE DATE IN TODAY'S FILE","")
Alternative to saving query object is to open a recordset in VBA and check its Recordcount or field value.