After a prātipadika (Nominal Stem) being in the Locative -7th case in construction denoting time, an affix comes in the sense of 'being then due'; provided that the thing due be 'debt'.,
Thus मासे देयमृणं = मासिकं 'a debt due in a month'. आर्द्धमासिकम्, सांवत्सरिकम् ॥ Why do we say ऋणे 'it being a debt'. Observe मासेदेया भिक्षा where no affix is added.,