I have a holographic will that is dated, but without the year, is this valid?

Yes, an undated will is valid so a partially dated will is fine too. If there is another will then you would need evidence to prove which is newer. For example, I had an undated holographic will, in a case, and it made reference to some addition to the house. That addition had been made in a certain year so they knew the will was from that year or later. It trumped an earlier dated will. I hope this helps.