calendar-nongregorian-visible-p is a compiled Lisp function in `calendar.el'.

(calendar-nongregorian-visible-p MONTH DAY TOABS FROMABS SWITCH)

Return non-nil if MONTH, DAY is visible in the calendar window.
MONTH and DAY are in some non-Gregorian calendar system. The
functions TOABS and FROMABS convert that system to and from
absolute, respectively. SWITCH is a function that takes a single
argument (a local month number). It applies when the local year
changes across the calendar window, and returns non-nil if the
specified month should be associated with the higher year.
Returns the corresponding Gregorian date.