do not apply twice the decimal part during date conversion to gregorian calendar

Created originally on Bitbucket by ericgazoni (Eric Gazoni)

Was already merged in Bitbucket before import, marked as merged by the import user

there is a miscalculation happening when you're nearing midnight (e.g. 23:59:59.999999), but not quite there yet (e.g. the decimal part of the julian date rounds to the next day), jdcal will round to the next day, but openpyxl would apply the full decimal part, adding 23:59:59.99999.

This fix neutralizes openpyxl when jdcal has already done the "jump"