Time card help!!!!!!!!!!!!!

My company is using handouts for our time cards and we take a picture and send it out to the office at the end of the week. I'm tired of printing them out or running out of sheets and would like to do this on the go with my iPhone and send the pdf to my boss. I have the formula figured out for time start and time stop total hours. Where I ran into the problem is I want to be able to enter in .5 in the lunch cell and have this automatically be subtracted when I do so. If I don't add the .5 than my hours stay they same.

Example: My shift starts at 7:00 am. Ends at 3:30. This includes lunch. My company wants us to but in .5 for the lunch section and this gives us a value of 8 in the TOTAL CELL for the day. I sometimes take lunch and sometimes just work through the day without one. So I would like to be able to just enter in the time I start, stop, and if I take a lunch I add .5 and have that calculated in. If I leave it blank than no subtraction is needed. I've been trying for hours and can't come up with a good solution. The time format is giving me lots of trouble. Can anyone help me solve this?

TIME START CELL: X11
TIME STOP CELL: Y11
TOTAL CELL: Z11

LUNCH CELL: U11

To get the total I have the formula as =(y11-x11)*24, which gives me the format that my company likes to use.


Also, when I copied this formula all the way down to the bottom of the totals column it works perfectly but is their away to get rid of the "0.00" value when i have no values for TIME START and TIME STOP? I still want it to function but just want more white space to make this easier to read.

If you just want to enter 3:30, rather than 3:30 PM, for stop time, try the following:
=(IF(Y11<X11,Y11+12/24,Y11)-X11)*24-U11
Explanation: Excel registers a 3:30 entry as 3:30 AM, so you trick it into calculating afternoon hours as PM by adding 12 hours.

To get rid of the "0.00" values...
=IF(OR(X11="",Y11=""),"",(IF(Y11<X11,Y11+12/24,Y11)-X11)*24-U11)

NICE!!!! Everything works very well now, just how I like it. Thank you