Narada Purana — Purva Bhaga, Shloka 49
Tithi-Nirṇaya for Vratas: Ekādaśī Rules, Saṅkrānti Punya-kāla, Eclipse Observances, and Prāyaścitta
एकादशी कलामात्रा विद्यते द्वादशीदिने । द्वादशी च त्रयोदश्यां नास्ति वा विद्यतेऽथवा ॥ ४९ ॥
ekādaśī kalāmātrā vidyate dvādaśīdine | dvādaśī ca trayodaśyāṃ nāsti vā vidyate'thavā || 49 ||
若在十二日（Dvādaśī）之日，十一斋日（Ekādaśī）仅存片刻；而十二日相对于十三日（Trayodaśī）又或全无，或仅微微出现，则应依此等“失日”（tithi 缺失）之相，判定如法的受持。
Sanatkumara (in dialogue with Narada)
Vrata: Ekādaśī-vrata
Primary Rasa: shanta
Secondary Rasa: adbhuta
FAQs
It emphasizes that Ekādaśī-vrata is not casual but must be aligned with the true lunar tithi; even a “fractional” presence of Ekādaśī or loss of Dvādaśī affects the dharmic correctness of fasting and pāraṇa.
By insisting on precise Ekādaśī–Dvādaśī observance, the text frames bhakti as disciplined devotion—serving Hari through correctly timed vrata and pāraṇa rather than mere intention alone.
Vedāṅga Jyotiṣa/Pañcāṅga-based tithi analysis—especially tithi-kṣaya (loss) and kalā-mātrā (minute-duration) rules used to decide the proper day for fasting and the correct window for Dvādaśī pāraṇa.