Hello C.,
I agree with everyone who said this is pretty normal for this age. I also agree she is likely too young for time outs but that is just based on my own experience. I think advice given should be taken with a grain of salt because all kids at one age will have so many variations on ability, understanding and personality. It is hard to know what will work for your daughter.
Consistency is the most important part of the equation. I've got a 17 month old here at home and he still hits on occasion. It doesn't matter if it's once or 100 billion times, each and every time you have to do the same thing that you have decided to do whether it's talking to her or time-out, or whatever.
Also, ask everyone taking care of your daughter to be on the same page as far as how to react to hitting (once you've chosen a course of action). I have made sure my sitter and my husband both do it similarly to what I do. It made a big difference because I had found out the sitter was just laughing and he was making a game out of it (because she didn't know what else to do!).
I have found now that he is older, when I say "Ouch! No hitting; that hurts Mommy" he actually comes over and kisses the boo-boo. I also replace his hitting behavior with a reminder on what "gentle" or "nice" is like, and then move quickly to another activity. I thought this particular child would NEVER learn to "do nice" but after a million times, lo and behold...he did. :)
They can learn! Just be consistent, and enjoy your daughter.