Identifying Start And End Dates In A Series

i am trying to both identify the start date and the end date of a series in excel.

if you will, picture a spreadsheet with two rows with data. the top row is comprised of dates in mm/dd/yy (end of month) format. the bottom line is percentages . . . a corresponding % for each date directly above. these percentages add to 100%. in two separate formulas, I would like to identify:

1. the date in which the first %>0 occurs (start date)
2. the date in which the cumulative addition of all months %'s equate to 100% or the date in which the last %>0 occurs (end date)

keep in mind many if not most of the %'s will be 0%.