Delay in giving Zakaat is a sin
When 1 year passes on wealth which meets the prescribed criterion then Zakaat becomes Compulsory. Delaying in giving Zakaat for 1 more year after it becomes Compulsory without any valid reason is a sin.
Zakaat is one of the obligations of Islam and a great religious responsibility. We should hasten towards it. Wealthy Muslims should give Zakaat as soon as the year ends. No one can be heedless about death. If death comes before one gives it, then one will be considered to have abandoned an obligation of Islam. As given in Radd Ul Muhtaar, Vol. 2, Kitab Uz Zakaat.