New function GncDateTime::format_zulu(), makes time string in GMT.

Required to ensure that date-times stored in SQL backend are in GMT for
backward compatibility.

src/libqof/qof/gnc-datetime.cpp

@@ -71,6 +71,7 @@ public:
     void today() { m_greg = boost::gregorian::day_clock::local_day(); }
     ymd year_month_day() const;
     std::string format(const char* format) const;
+    std::string format_zulu(const char* format) const;
 private:
     Date m_greg;
 };
@@ -147,6 +148,7 @@ public:
     struct tm utc_tm() const { return to_tm(m_time.utc_time()); }
     std::unique_ptr<GncDateImpl> date() const;
     std::string format(const char* format) const;
+    std::string format_zulu(const char* format) const;
 private:
     LDT m_time;
 };
@@ -256,6 +258,18 @@ GncDateTimeImpl::format(const char* format) const
     return ss.str();
 }
 
+std::string
+GncDateTimeImpl::format_zulu(const char* format) const
+{
+    using Facet = boost::posix_time::time_facet;
+    std::stringstream ss;
+    //The stream destructor frees the facet, so it must be heap-allocated.
+    auto output_facet(new Facet(format));
+    ss.imbue(std::locale(std::locale(), output_facet));
+    ss << m_time.utc_time();
+    return ss.str();
+}
+
 /* =================== Presentation-class Implementations ====================*/
 /* GncDate */
 GncDate::GncDate() : m_impl{new GncDateImpl} {}
@@ -337,3 +351,9 @@ GncDateTime::format(const char* format) const
 {
     return m_impl->format(format);
 }
+
+std::string
+GncDateTime::format_zulu(const char* format) const
+{
+    return m_impl->format_zulu(format);
+}

src/libqof/qof/gnc-datetime.hpp

@@ -157,10 +157,19 @@ public:
  *  presented. Code letters preceded with % stand in for arguments;
  *  most are the same as described in strftime(3), but there are a few
  *  differences. Consult the boost::date_time documentation.
- *  @return a std::string containing a representation of the date
- *  according to the format.
+ *  @return a std::string containing a representation of the date and time in
+ *  the locale's time zone according to the format.
  */
     std::string format(const char* format) const;
+/** Format the GncDateTime into a std::string in GMT
+ *  @param format: A cstr describing the way the date and time are
+ *  presented. Code letters preceded with % stand in for arguments;
+ *  most are the same as described in strftime(3), but there are a few
+ *  differences. Consult the boost::date_time documentation.
+ *  @return a std::string containing a representation of the date and time in
+ *  GMT (timezone Z) according to the format.
+ */
+    std::string format_zulu(const char* format) const;
 
 private:
     std::unique_ptr<GncDateTimeImpl> m_impl;

src/libqof/qof/test/gtest-gnc-datetime.cpp

@@ -77,6 +77,13 @@ TEST(gnc_datetime_functions, test_format)
     EXPECT_EQ(atime.format("%d-%m-%Y"), "13-11-2045");
 }
 
+TEST(gnc_datetime_functions, test_format_zulu)
+{
+    GncDateTime atime(2394187200); //2045-11-13 12:00:00 Z
+    //Date only to finesse timezone issues. It will still fail in +12 DST.
+    EXPECT_EQ(atime.format_zulu("%d-%m-%Y %H:%M:%S"), "13-11-2045 12:00:00");
+}
+
 //This is a bit convoluted because it uses GncDate's GncDateImpl constructor and year_month_day() function. There's no good way to test the former without violating the privacy of the implementation.
 TEST(gnc_datetime_functions, test_date)
 {