Hi Gary,
You may be making things very difficult to work with by
someone who is good at entering data.   Also Data Validation
can be defeated by pasting, which also wipes out the validation.

To read about "Data Validations"  at Debra Dalgleish's site
 http://www.contextures.com/tiptech.html
You might also check  Dick Kusleika's "Daily Dose of Excel"
 http://www.google.com/search?q=site%3Adailydoseofexcel.com+validation&num=100

  In the date cell, use Data - Validation - Custom;  =A2<>""
   Clear the "Ignore blank" box.     [Earl's reply]

A2: client name
B2: Date      would have the cell validation  =A2<>""
C2: Hours    would have the cell validation  =B2<>"" or perhaps =NOT(ISBLANK(B2))
A3: client name     might have a cell validation checking C2

with very limited description of form, that is the best that
I can add, but  you also have  data beyond C2 that would be optional
so this could be very complicated for you to design and maintain
and be difficult for data entry if not done properly.

But your new requirement to not enter a new client before the previous
client has been filled in might be a real breaker.  ("Each line must be
filled in prior to proceeding to the next line.")

BTW, you should be replying to Earl's reply, not to your original
post in the thread.  That way Earl and others, including yourself,
would see that you are replying to his reply (which would also still
be under the thread you began).  It would also better reflect the
manner in which you are modifying your question.
---
HTH,
David McRitchie, Microsoft MVP - Excel
My Excel Pages:  http://www.mvps.org/dmcritchie/excel/excel.htm
Search Page:        http://www.mvps.org/dmcritchie/excel/search.htm