implement a flag (in this case, an enumeration) that allows control over the type; we make it an enum for possible expansions of time abstractions that might make it into the C++ standard in the future (see Howard Hinnant's date/timezone library) or might be usefully-available from the OS at some point in time

include/spdlog/async_logger.h

@@ -71,7 +71,7 @@ public:
 protected:
     void _sink_it(details::log_msg& msg) override;
     void _set_formatter(spdlog::formatter_ptr msg_formatter) override;
-    void _set_pattern(const std::string& pattern) override;
+    void _set_pattern(const std::string& pattern, pattern_time ptime) override;
 
 private:
     std::unique_ptr<details::async_log_helper> _async_log_helper;

include/spdlog/common.h

@@ -106,6 +106,15 @@ enum class async_overflow_policy
     discard_log_msg // Discard the message it enqueue fails
 };
 
+//
+// Pattern time - specific time getting to use for pattern_formatter.
+// local time by default
+//
+enum class pattern_time
+{
+	local, // use variant of time similar to std::localtime for std::tm
+	utc // use variant of time similar to std::gmtime for std::tm
+};
 
 //
 // Log exception

include/spdlog/details/async_logger_impl.h

@@ -76,9 +76,9 @@ inline void spdlog::async_logger::_set_formatter(spdlog::formatter_ptr msg_forma
     _async_log_helper->set_formatter(_formatter);
 }
 
-inline void spdlog::async_logger::_set_pattern(const std::string& pattern)
+inline void spdlog::async_logger::_set_pattern(const std::string& pattern, pattern_time ptime)
 {
-    _formatter = std::make_shared<pattern_formatter>(pattern);
+    _formatter = std::make_shared<pattern_formatter>(pattern, ptime);
     _async_log_helper->set_formatter(_formatter);
 }
 

include/spdlog/details/logger_impl.h

@@ -53,9 +53,9 @@ inline void spdlog::logger::set_formatter(spdlog::formatter_ptr msg_formatter)
     _set_formatter(msg_formatter);
 }
 
-inline void spdlog::logger::set_pattern(const std::string& pattern)
+inline void spdlog::logger::set_pattern(const std::string& pattern, pattern_time ptime)
 {
-    _set_pattern(pattern);
+    _set_pattern(pattern, ptime);
 }
 
 
@@ -316,9 +316,9 @@ inline void spdlog::logger::_sink_it(details::log_msg& msg)
         flush();
 }
 
-inline void spdlog::logger::_set_pattern(const std::string& pattern)
+inline void spdlog::logger::_set_pattern(const std::string& pattern, pattern_time ptime)
 {
-    _formatter = std::make_shared<pattern_formatter>(pattern);
+    _formatter = std::make_shared<pattern_formatter>(pattern, ptime);
 }
 inline void spdlog::logger::_set_formatter(formatter_ptr msg_formatter)
 {

include/spdlog/details/pattern_formatter_impl.h

@@ -311,7 +311,15 @@ class R_formatter SPDLOG_FINAL:public flag_formatter
 {
     void format(details::log_msg& msg, const std::tm& tm_time) override
     {
-        pad_n_join(msg.formatted, tm_time.tm_hour, tm_time.tm_min, ':');
+		msg.formatted << (tm_time.tm_year + 1900);
+		msg.formatted << '-';
+		msg.formatted << tm_time.tm_mon;
+		msg.formatted << '-';
+		msg.formatted << tm_time.tm_mday;
+	
+		msg.formatted << ' ';
+		
+		pad_n_join(msg.formatted, tm_time.tm_hour, tm_time.tm_min, ':');
     }
 };
 
@@ -324,7 +332,6 @@ class T_formatter SPDLOG_FINAL:public flag_formatter
     }
 };
 
-
 // ISO 8601 offset from UTC in timezone (+-HH:MM)
 class z_formatter SPDLOG_FINAL:public flag_formatter
 {
@@ -495,12 +502,13 @@ class full_formatter SPDLOG_FINAL:public flag_formatter
 ///////////////////////////////////////////////////////////////////////////////
 // pattern_formatter inline impl
 ///////////////////////////////////////////////////////////////////////////////
-inline spdlog::pattern_formatter::pattern_formatter(const std::string& pattern)
+inline spdlog::pattern_formatter::pattern_formatter(const std::string& pattern, pattern_time ptime)
+: _time(ptime), _pattern(pattern)
 {
-    compile_pattern(pattern);
+    compile_pattern(_pattern, _time);
 }
 
-inline void spdlog::pattern_formatter::compile_pattern(const std::string& pattern)
+inline void spdlog::pattern_formatter::compile_pattern(const std::string& pattern, pattern_time ptime)
 {
     auto end = pattern.end();
     std::unique_ptr<details::aggregate_formatter> user_chars;
@@ -670,7 +678,21 @@ inline void spdlog::pattern_formatter::format(details::log_msg& msg)
 {
 
 #ifndef SPDLOG_NO_DATETIME
-    auto tm_time = details::os::localtime(log_clock::to_time_t(msg.time));
+	auto tm_time = [this, &msg]()
+	{
+		switch (_time)
+		{
+		// it is always faster to put the most-common/default case first
+		case (pattern_time::local):
+			return details::os::localtime(log_clock::to_time_t(msg.time));
+
+		case (pattern_time::utc):
+			return details::os::gmtime(log_clock::to_time_t(msg.time));
+		
+		default:
+			return details::os::localtime(log_clock::to_time_t(msg.time));
+		}
+	}();
 #else
     std::tm tm_time;
 #endif

include/spdlog/formatter.h

@@ -29,15 +29,16 @@ class pattern_formatter SPDLOG_FINAL : public formatter
 {
 
 public:
-    explicit pattern_formatter(const std::string& pattern);
+    explicit pattern_formatter(const std::string& pattern, pattern_time ptime = pattern_time::local);
     pattern_formatter(const pattern_formatter&) = delete;
     pattern_formatter& operator=(const pattern_formatter&) = delete;
     void format(details::log_msg& msg) override;
 private:
     const std::string _pattern;
+	const pattern_time _time;
     std::vector<std::unique_ptr<details::flag_formatter>> _formatters;
     void handle_flag(char flag);
-    void compile_pattern(const std::string& pattern);
+    void compile_pattern(const std::string& pattern, pattern_time ptime);
 };
 }
 

include/spdlog/logger.h

@@ -66,7 +66,7 @@ public:
     void set_level(level::level_enum);
     level::level_enum level() const;
     const std::string& name() const;
-    void set_pattern(const std::string&);
+    void set_pattern(const std::string&, pattern_time = pattern_time::local);
     void set_formatter(formatter_ptr);
 
     // automatically call flush() if message level >= log_level
@@ -82,7 +82,7 @@ public:
 
 protected:
     virtual void _sink_it(details::log_msg&);
-    virtual void _set_pattern(const std::string&);
+    virtual void _set_pattern(const std::string&, pattern_time);
     virtual void _set_formatter(formatter_ptr);
 
     // default error handler: print the error to stderr with the max rate of 1 message/minute