Satyavratasmriti 1.9: Jayasīṃhakalpadruma · Pratāpārka · Nirṇayasindhu

1.9

rāśidvayaṃ yatra māse saṃkramate divākaraḥ | nādhimāso bhaved eṣa malamāsas tu kevalam

The month in which the sun transits into two zodiacal signs / is not an intercalary month (adhimāsa), but merely an impure month (malamāsa).

A month in which the sun passes through two signs is not an intercalary month (adhimāsa) but only a 'dirty' month (malamāsa).