The question I have is the use of the word "to" in the phrase "to December 28". Does the "to" definitely include December 28th, or is it (as I think it is) ambiguous? The way it reads, I feel it is ambiguous as to whether hours worked and record on December 28th will be included in the live payroll processing date of December 29th. I think a less-ambiguous word choice would be "through" instead of "to".
"To" itself is clear enough as you can see in the definitions from the dictionary, but I don't disagree that an average person might get confused about this, if they don't really have more context about the sentence:
10. b): until and including a particular time or date:
They stayed from Friday night to Sunday morning.
I'll be on duty from 8 am to 10 pm.
time: during and to the end of a period of time
[Both from Longman Dictionary of Contemporary English]
However, an alternative way to make sure it's clear to everyone, is to say it this way:
The first live processing date is December 29, 2014 for the pay period of December 22, 2014 to the end of December 28, 2014.
both to and through may be ambiguous to some people. It is generally accepted that the last day is included; specifically when concerned with expiration dates.
However the only way to be precisely understood is to say something like either:
up to and including "the last date"
up to but not including "the last date"
Either one can be used so long as the proper date is given with them. You will find this is used often in legal documents when the writer wants to be most specific.