Formula for finding last delivery date and time

Hello,
My company receives shipments every Wednesday at 4pm and Saturday at 4pm.
I have a list of event times in column A and in column B, I would like a formula to list when the the shipment arrives for that row's event.
In my attached spreadsheet, I've included some examples, with the answer I need in column C

So for an event on Wednesday, June 12 @ 8am, items were delivered on the past Saturday, June 9 @ 4pm
But an event later that day, Wednesday, June 12 @ 7pm, the items were delivered only a few hours earlier, Wednesday, June 12 @ 4pm.

Also a brief explination as to why this works would be helpful for future problems of mine.
This type of time math always confuses me!
Thanks in advance for the help! and let me know if I need to explain my problem differently.
Joe

So why is your very first answer Sunday June 3rd, 2012??

Because I am a fool, it should be Saturday June 2.

Sorry about that!!

Because I am a fool, it should be Saturday June 2.

Sorry about that!!

Try this - I haven't fully tested it.

=IF(AND(OR(WEEKDAY(A2)={4,6}),HOUR(A2)<16),INT(A2),INT(A2)-CHOOSE(WEEKDAY(A2),1,2,3,4,1,2,3))+16/24

Good stuff. Glad to hear it works for you. Now the explanation:
=IF(AND(OR(WEEKDAY(A2)={4,7}),HOUR(A2)>16),INT(A2),INT(A2)-CHOOSE(WEEKDAY(A2),1,2,3,4,1,2,3))+16/24

The first part: IF(AND(OR(WEEKDAY(A2)={4,7}),HOUR(A2)>16) says this
If the date in A2 is a Wednesday or Saturday ({4,7}) and the time is after 4pm (HOUR()>16) then ....
If that part is true the result for that condition is the date (without the time) shown in A2
If the first part is false then a variable # will be subtracted from the date in A2. That variable # is determined by the CHOOSE() function and is dependent upon the WEEKDAY() value of A2 - Sunday is 1, Monday is 2, Tuesday is 3......
For example if A2 is a Tuesday you want to backtrack 3 days to get to Saturday - you'll see that the 3rd option within the CHOOSE() is a 3.
So the IF() function determines the delivery date and then 4pm is added to it (the 16/24 part - 16 being the 16th hour of 24 hour day)
The end result is Date + 4pm

Thank you cutter!

You're welcome. Clear as mud????