Good question, frequently asked by buyers. As it was mentioned in other answers, there is no connection between the two in terms of the approach used to determine them.
But to answer your question as it was asked, it could be the the date of the last sale.
In Tompkins County, Department of Assessment typically matches assessed value to the purchase price of the last sale of the property soon after the sale. Therefore, if property was sold recently, assessed value is somewhat the same, but if property wasn't for sale for long time, assessed value can be substantially lower than current market value of the property, given, of course, property values appreciate. Hope it helps.