This sounds like a lookup problem. Do you want to return the date that matches to the maximum of your column of numbers? If so, and if your column of numbers appears to the left of your dates, try:
=VLOOKUP(MAX(range_of_numbers),table,col_index,FALSE)
table defines you range of numbers and dates
col_index defines the column within the table that contains your dates
FALSE means that you require an exact match which must be possible since your lookup value must be within the lookup range.
See Excel help for more info.