mRNA decapping enzyme that specifically removes the nicotinamide adenine dinucleotide (NAD) cap from a subset of mRNAs by hydrolyzing the diphosphate linkage to produce nicotinamide mononucleotide (NMN) and 5' monophosphate mRNA. The NAD-cap is present at the 5'-end of some RNAs; in contrast to the canonical N7 methylguanosine (m7G) cap, the NAD cap promotes mRNA decay. Preferentially acts on NAD-capped transcripts in response to nutrient stress. Also acts on free nicotinamide adenine dinucleotide molecules: hydrolyzes NAD(H) into NMN(H) and AMP, and NADPH into NMNH and 2',5'-ADP. May act to regulate the concentration of peroxisomal nicotinamide nucleotide cofactors required for oxidative metabolism in this organelle. Regulates the levels of circadian clock components PER1, PER2, PER3 and CRY2 in the liver 