sysIsValidDate

Checks whether the specified date is valid according to the Gregorian calendar. Returns 1 if the date is valid, 0 if invalid. Validates that year is between 1 and 9999, month is between 1 and 12, and day is within the valid range for that month (accounting for leap years).

Uses sysDaysInMonth internally to determine valid day ranges, which automatically handles February in leap years (29 days) and non-leap years (28 days). Useful for validating user input before performing date calculations.