Muhurat Martanda · by Narayana Daivajna
Chapter 11 · The Sun's Sign Entry and the Months
संक्रांतिप्रकरणम्
ब्रह्माद्यैरिनमंडलांत उदितश्चांद्रस्त्वमांतः परैर्मासोऽसंक्रमणो द्विसंक्रमणको ज्ञेयोऽधिकोऽथो क्षयः । कर्माण्यन्यगतीनि नात्र तनुयाद्यज्जन्ममृत्यू क्षये तन्मासौ तिथिखंडयोश्चयमुखे भृत्येऽधिको गण्यते ॥ २ ॥
Brahma and others say a month ends when the Sun ends its round of a sign. Others take the lunar month, which ends at the new moon. A month with no sankranti (the Sun's entry into a sign) is the adhimasa (extra month). A month with two sankrantis is the kshayamasa (lost month). In these months, do not do rites that can be done at another time. Say a birth or a death falls in a lost month. Then its two months are kept in the two halves of the tithi (moon day). For interest and the like, and for wages, the extra month is counted.
Simple meaning: A month without a Sun entry is extra; with two, it is lost. Skip optional rites in them.