The affix ṭhañ comes optionally comes in the remaining senses after the words 1. niśā and 2. pradoṣa ,
This makes optional, where by sutra 4.3.11 ante, the ठञ् would have been obligatory. Thus नैशम् or नैशिकम् 'nocturnal'. प्रादोषम् or प्रादोषिकम् ॥,