The date of valuation is January 1st therefore; information presented during the appeal process should be based on the previous twelve months sales, listing of sales, and other data relevant to that specific time period.
Simply stating that property taxes are too high is not considered relevant information. You should establish in your mind what you think your property is worth. The best evidence of this would be the recent sale price of your property. The next best evidence would be recent sales prices of properties that are very similar to yours, the closer in proximity and similarity, the better the evidence. Another type of evidence is a recent appraisal of your property. Keep in mind, your evidence should be strong enough to substantiate your opinion of value.