QtCS2018 Date Time

Howard Hinnant's new civil calendar and timezone proposals have made it to the C++ 20 draft (N4750)

• Not implemented yet in any C++ Standard Library

Time Zone

• Why would we use this, if we have to keep our existing code?
  • Allows us to hook to a standard tzdb implementation without forcing an ICU dependency
  • Good for Windows (MS doesn't want us to use the registry DB anyway)
  • Good for iOS (no need for Apple's ICU-wrapper API)
• Can it be used without exceptions?
• We should add a backend for it when we can
  • Drop unneeded backends

Front-end API

• What do we do for QDate and QTime?
  • We should add conversion API for the new types as soon as we can
  • QT_HAS_INCLUDE(<chrono>), like QDeadlineTimer does for std::chrono::duration and std::chrono::time_point
• Is there anything missing in C++20 draft which would prevent a QDateTime refactor in the 2020s?
  • (Keeping QDateTime convenience API but replace the QDateTimePrivate backend)
  • Eddy is working with Howard to make sure everything is there
• Compilers don't have this yet, but should have it in 2019 or early 2020
  • But we don't think we can reimplement QDateTime using date.h for Qt 6
  • Too close to call, regarding BC guarantees
• Review QDateTime API:
  • Anything we want to deprecate anyway?
  • Anything that would be difficult to implement with date.h and we'd want to provide different API for?

Calendar systems

• Didn't have enough time to discuss this
• We have a contribution for QAbstractCalendar
  • We need a solution that enables code outside of Qt to implement their own calendars