Count unique records for a specific date

There are actually quite a few moving parts in this formula:
F1: =SUMPRODUCT(--ISNUMBER(1/((MATCH(B2:B20&$E$1,B2:B20&A2:A20,0)-1+ROW(A$2))=ROW(B2:B20))))

Basically, it looks for each first instance of Job#/Date combination
where the date matches the referenced date.

This section:
MATCH(B2:B20&$E$1,B2:B20&A2:A20,0) finds the matches.

and

this section:
-1+ROW(A$2)

just tweaks the results of the match section so it returns row numbers.

The first part: B2:B20&$E$1
combines each actual Job# with the reference date
in E1 (01-FEB-2010, in the example).
Note: the Excel date serial number for that date is 40210.

These are the Job#/Ref_Date pairs to find:
List_1
1040210
1040210
1040210
1140210
1240210
1240210
1340210
1440210
etc

These are the actual pairs of Job# and Date
from the Col_A/Col_B list:
List_2
1040210
1040210
1040210
1140210
1240211
1240211
1340211
1440211

so, the MATCH function looks for each List_1 item in List_2
and returns the list position.

In the example, the first 3 List_1 items (1040210) are found in the
first position of List_2...but, only the first instance matches
the row number, resulting in TRUE (which is converted to 1).

All non-first-matches result in FALSE (which is converted to 0)

Since those results are used in fraction denominators,
1/1 is a valid number
1/0 is an error (#DIV/0!)

The ISNUMBER function, When applied to the list of returned values (errors or 1's),
returns a 1 for each first instance and a 0 for each non-first-instance.

The SUMPRODUCT function adds up the first-instance values to return their count.

Perhaps not the clearest explanation, but that is definitely an advanced,
NOT a beginner, formula.

I hope that helps.