Calendars of the World is an application for Android devices that allows you to view the world through different calendars. In addition to the Gregorian Calendar, you have access to the Julian Calendar, the Hebrew Calendar, the Islamic Calendar, the Persian calendar, the Indian Civil calendar and the Bahá'í/Badí calendar (up to date with the new revisions since 2015).
The application is available on the Google Play Store in both ad-supported and paid versions.
Source code is not available for general use. However, if somebody needs the source code for a specific purpose (such as to review the code for the purposes of an employment or contract), I may be able to provide access to it on a confidential basis.