They are prosecuting me now over a last year matter on the ground that it has now become an offence this year
Such prosecution is illegal. Subject to the provisions of the constitution, a person shall not be convicted of a criminal offence unless that offence is defined and the penalty for the offence is prescribed in a written law. Such offence and conviction must be known to law.
Moreover, conviction on retro-active legislation is unlawful. This means that a person shall not be convicted on account of an act which at the time it took place, did not constitute an offence and no penalty shall be imposed for any criminal offence heavier than that in force at the time the offence was committed.
However, if the offence you are being charged now was an offence committed years ago and constituted a crime when it was committed without timeframe which we lawyers call statute-barred, you cannot invoke retro-active legislation as a defence.
 –Section 36 (12) of the 1999 Constitution See Aoko Vs. Fagbemi (1961) 1 All NLR 400; A.G.F. Vs. Isong (1986) 1QLRN 75
 Section 36 (8) of 1999 Constitution