So the linking dimension is a date? I see the values for the date in the excel file are like July 14, what do the values look like in the database table. For example, what value(s) are you trying to join to the row that says 'July 14' ?
Yes, they are dates.
Oracle example: 22/04/2014 04:40:00 p.m.
Can you provide some sample records for Oracle?
It's not clear to me if you're aggregating Oracle to the month level (that's what it sounds like in your first post) or if you're retrieving records from Oracle at the lowest level (like it sounds like in your 2nd post).
If your data in Oracle contains records which are at the day and hour level, you would need to create a calculated field in the Oracle source in Tableau which strips out the time from the date, then join on that to the excel source.
My date already had the time stripped, BUT i just created a string with YEARMONTH (ie, 20154 for April 2005), made sure the column in my excel had also that string for each month and that settled the issue. I hate dealing with Oracle's dates.