We currently have no plans to develop for iOS.
We looked into the details of how to develop a calendar app for iOS, but it seems that there is (at least) one unsolvable problem (or actually it is solvable but it breaks Apple's guidelines, and thus risks being banned from the app store).
The development costs (plus required marketing costs to gain any visibility in a tight market like calendars for iOS) are too high to take such a risk for a small indie studio like Tapir Apps.
If you are an experienced iOS developer and would want to develop/license aCalendar for iOS, contact us.
If you are searching for a calendar app for iOS, my favorite German tech-blogger recommends fantastical or calendars 5.