Variable Lookup in a table

Hi Guys,

I would appreciate it if someone could help me with some sort of lookup function. I have attached my spreadsheet in which I am trying to do the lookups.

What I am trying to do is:

On the MONTHS tab, I am trying to lookup information from the UNITS tab. I have put into the MONTHS tab some formula purely to illustrate what I need the answer to be. Essentially, the lookup should use the date from the MONTHS tab (which will always be month end) as the value for the lookup on the UNIT tab. The complication is for example, in September on the MONTH tab, the lookup should use 30 September, but there are three September dates on the UNIT tab.

Under the DEFERRED & LIABILITY columns, I need to lookup the highest date in the September range. However, in the INCOME and INTEREST columns, I need the lookup to find September and add all the income and all the interest for various September dates (I have just used the SUM formula for illustration, but this won't work when the dates are variable.

I hope my explanation is a bit clearer than mud! I have tried to solve the problem, but the standard VLOOKUP and INDEX MATCH don't seem to work.

Thank you.