Date & Time Calculator (9 in 1)
Best Time Converter
Developer: Thomas Tsopanakis
Purchase date: 02.06.2016 for 0.99 EUR
Rating: ***** 5 stars
System requirements: iOS 10.0 or later. Compatible with iPhone, iPad and iPod touch.
Functionality: Date and Time Calculator (9 in 1) is the perfect time calculation tool. It is by far the best and most accurate time calculator for iOS!
In the meantime I checked again if there are more apps of this kind and quality at the App Store (no, not really).
Conclusion: highly recommended
Best solution at the App Store for date/time calculation.