Is the default_date() defined below thread safe according to C++11 Standard?

This is another example extracted from B.Stroustrup's new book "TCPL", more specifically from itsSection 16.3 Concrete Classes. As this is a rather long section, I'll summarize the points of interest below.

namespace Chrono {
    enum class Month { jan=1, feb, mar, apr, may, jun, jul, aug, sep, oct, nov, dec };
    class Date {
    public:
        class Bad_date { };
        explicit Date(int dd ={}, Month mm ={}, int yy ={});
        int day() const;
        Month month() const;
        int year() const;
        Date& add_year(int n);
        Date& add_month(int n);
        Date& add_day(int n);
     private:
        bool is_valid();
        int d, m, y;
     };

     bool is_date(int d, Month m, int y);
     bool is_leapyear(int y);
     bool operator==(const Date& a, const Date& b);
     bool operator!=(const Date& a, const Date& b);
     const Date& default_date();
     ostream& operator<<(ostream& os, const Date& d);
     istream& operator>>(istream& is, Date& d);
} // Chrono

Constructor definition:

Date::Date(int dd, Month mm, int yy)
:d{dd}, m{mm}, y{yy}
{
    if (y == 0) y = default_date().year();
    if (m == Month{}) m = default_date().month();
    if (d == 0) d = default_date().day();
    if (!is_valid()) throw Bad_date();
}

Helper function default_date() definition:

const Date& Chrono::default_date()
{
    static Date d {1,Month::jan,1970};
    return d;
}