The Islamic Calendar has had support in Java since 8 introduced the Date and Time API. I don't know if OS support was around, Wikipedia is sparse on the topic.

[#] - https://en.wikipedia.org/wiki/Islamic_calendar#Computer_supp...

At least the Saudi Arabian bank that I've worked with did not use a lunar calendar on their servers. At least as far as I know, and I'm sure it would have come up, our application doing lots of date/time manipulation and scheduling.

Calendar on Mac, and I'm pretty sure on iOS too, lets you show the Islamic, Chinese, or Hebrew calendar along with the Gregorian calendar

They did and still do- optionally. But the system probably uses Gregorian underneath it all, using the Hijri calendar just for display and maybe file timestamps (been a while since I had mine set to Hijri so I'm not certain).

It can be enabled pretty easily in Windows 10: http://windowsclan.com/switch-to-hijri-calendar-in-windows-1...