[Development] QtCS 2018 - Date/time session

Thiago Macieira thiago.macieira at intel.com
Mon Jun 11 14:59:55 CEST 2018

Link: https://wiki.qt.io/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 
** 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 
** 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 

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center

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