Pañcāṅga — tithi · vāra · nakṣatra · yoga · karaṇa

Siddhānta path (SS mool). Mean longitudes from the Sūrya-siddhānta revolutions, then manda/śīghra corrections to true (sphuṭa) longitudes, and the siddhāntic ayanāṃśa (trepidation, ±27°) reduced to nirayana. Its zero point and planetary theory are the classical text's.
Drik path (TKS). Apparent positions from JPL DE kernels (light-time, aberration, deflection, IAU 2006/2000B), tropical ecliptic-of-date, then nirayana via our own TKS Ayanāṃśa — true Chitrāpakṣa (Spicā defined at nirayana 180°, from Hipparcos ICRS + proper motion, not a hard-coded constant). Official Lahiri (ICRC 1956) is shown alongside for comparison.
Why they differ. Two independent sources of difference: (i) the ayanāṃśa (TKS Ayanāṃśa ≈ 24.22° vs SS trepidation ≈ 26.24° here; Lahiri ≈ 24.23°), and (ii) the planetary theory (modern N-body/DE vs siddhāntic series). Small λ differences can flip a limb near a boundary; the derivation below uses nirayana longitudes throughout.
Derivations. tithi = (λ☾−λ☉)/12°; karaṇa = (λ☾−λ☉)/6°; nakṣatra = λ☾/(13°20′); yoga = (λ☉+λ☾)/(13°20′); rāśi = λ/30°; vāra from the local Julian day; saṅkrānti = the instant the Sun's nirayana λ crosses a 30° boundary. 60″ = 1 kalā · 60 kalā = 1°