TTVs from the ground-based follow-up campaign of P23 (black circles) including the high-precision Spitzer observations (gray squares), and our new Palomar/WIRC follow-up observations (orange stars), with 100 random posterior draws from our damped eccentricity TTV model (red) and free eccentricity TTV model (blue) for LP 791-18 c (top panel) and d (bottom panel). Our Palomar/WIRC timing measurements for LP 791-18 c are the most precise TTV observations of this system so far, a factor of 1.5–3× more precise than Spitzer, and drive the TTV model constraints for LP 791-18 d.