QDateTime::nullJd usage of numeric_limits

QDateTime::nullJd is defined as follows

  static inline qint64 nullJd() { return std::numeric_limits<qint64>::min(); }

With Microsoft VC++ this causes a compiler warning unless compiled with /DNOMINMAX, this isnt always an option when working with legacy code. Therefor I would suggest that QDateTime::nullJd is slightly rewritten:

  static inline qint64 nullJd() { return (std::numeric_limits<qint64>::min)(); }