I am attempting to build an report that shows the number of tickets submitted during each half hour interval of the day. I created a formula that pulls out the interval each ticket falls under. However, now I need the show '0' for those intervals where no tickets were submitted. I have tried this:

If isnull({Date.Field}) then 0 else
Time (Hour ({SRG_Ticket_Information_Customer.CompleteDate})
& ':'&
iIf( minute ({SRG_Ticket_Information_Customer.CompleteDate}) < 30, '00' ,'30'))

I get the error:
A number is required here.

Also,
Once I get this formula correct, I need the number of tickets assigned, Completed and Finished all grouped by each 48 half hour intervals of the day. I have been thinking once I get this formula, I can pull everything into Access and build a Crystal report from the queries.

Please HELP!

What happen here is that you try to return two different type of data with the same funtion.
Why don't you try '0' instead of 0:
If isnull({Date.Field}) then '0' else

When I did this, I received a message that a number was required.

if isnull({SRG_Ticket_Information_Customer.CompleteDa te}) then '0 '
else
Time (Hour ({SRG_Ticket_Information_Customer.CompleteDate})
& ':'&
iIf( minute ({SRG_Ticket_Information_Customer.CompleteDate}) < 30, '00' ,'30'))