The site originally held the largest public baths in the city and were completed in 306. However, over time these baths fell into ruin. A Cardinal acquired the site in 1533 when it was used again, this time to grow gardens amongst the ruins.
Work started on converting the baths to the church that Michelangelo had designed by 1563. After that, alternations have been done to the church including in 1702 when a sundial was placed on the floor. The sundial was important as its function was to check the validity of the new Gregorian calendar. The validity of the calendar was important in relation to the Spring Equinox, since the date of Easter depended on it.
If you are in Rome, it is definitely worth visiting the modern church and the remnants of the baths. If you visit, give yourself at least 1 hour to 1.5 hours so as not to be rushed.
Entrance