In reply to G. Tiger, Esq.:
Glad it did what you wanted - as pointed out, it may not be entirely accurate since it assumes your working day has the same number of hours before and after midday, eg. 08:00 to 16:00.
How it works:
NETWORKDAYS(D64,D65)-2 : Calculate the number of working days between your start and end dates, subtract 2 to discard the start day and the end day.
1-(D64-INT(D64)) : Calculate the fraction of the start day, since dates in Excel are just numbers where the integer part is the number of days since 1.1.1900 and the decimal part is the fraction of the current day (ie: the time) then D64 - INT(D64) strips away the date. Subtracting from 1 gives you how much of the day is remaining.
D65-INT(D65)) : The same as above for the end day but this time you want how much of the day has elapsed.
SUM() : Add the 3 values together to get the total days. Could just have used '+' but the modular approach using SUM might make it easier to understand!