Customer Saved Search Associated to a Certain Sales Rep and Display Duration of Time Entries Billed Against each Customer for the Previous Month

Solution:

1. Navigate to Lists> Search> Saved Searches> New

2. Select Customer

3. Under the Criteria tab> Standard subtab:
--Sales Rep = is John Doe

4. Under the Results tab> Columns subtab:
--Name, summary type = Group
--Time fields : Duration (Decimal), summary type = Sum
--Formula (Numeric) = case when (to_char({time.date},'MON-YYYY') = to_char (add_months({today},-1),'MON-YYYY'))  then {time.durationdecimal} else 0 end

Note:  Explanation of Column Fields:
Time: Duration (Decimal) field,  will give you the sum of the duration of ALL time entries billed against this customer
The formula (numeric) field  will give you the duration of all time entries billed against this customer for the previous/last month

5. Click Preview.