How do I calculate time in excel

I am doing a spreadsheet for my boss for hours worked. For example I would enter 1.4 for 1hr and 40 minutes and the 1.2 for 1hr 20 minutes. I know it is backwards but they bill out in 10 minute increments. How do I get excel to add 1.4 and 1.2 so it equals 3 hrs.

Or for example 10 minute = 0.17 and 20 minutes = 0.33 but when excel adds 0.17+0.17 using the SUM formula it equals 0.34 instead of 0.33.
I'm frustrated and at a loss. Any advice would be appreciated.

Hi, welcome to the forum

What you need to understand about dates and times in excel is...

a date is just a number representing the number of days passed since 1/1/900...and then formated in a way that we recognise as a date. So, for instance, today (Sun Jul 2015) is actually 42190

Time is actually a decimal part of 1 (day), so 06:00 AM is 0.25, 12 noon is 0.5 and 18:00 (or 6 PM) is 0.75

So, if you enter the times as real times, then the calc becomes easy

	B	C	D
1	Time 1	Time 2
2	1:40	1:20	3:00

D2=C2+B2

=SUM(B5:AF5)
This is a little shorter