Replace abseil date code with Hinnart's date.h

Howard Hinnart's date.h is the library that was accepted as C++20
date/calendar support, so this is essentially a backport of C++20 date
time support.

(It does support timezone support, but requires more of the library and
that seems like overkill for what we need; this just prints UTC
timestamps instead, which need only a header-only include).

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "external/optional-lite"]
 	path = external/optional-lite
 	url = https://github.com/martinmoene/optional-lite.git
+[submodule "external/date"]
+	path = external/date
+	url = https://github.com/HowardHinnant/date.git

CMakeLists.txt

@@ -255,6 +255,7 @@ if(SUBMODULE_CHECK)
     check_submodule(external/cxxopts)
     check_submodule(external/ghc-filesystem)
     check_submodule(external/optional-lite)
+    check_submodule(external/date)
   endif()
 endif()
 
@@ -268,6 +269,7 @@ add_subdirectory(external/nlohmann EXCLUDE_FROM_ALL)
 add_subdirectory(external/cxxopts)
 add_subdirectory(external/ghc-filesystem)
 add_subdirectory(external/optional-lite)
+add_subdirectory(external/date)
 
 if(ANDROID)
   list(APPEND LIBS log)

external/date

@@ -0,0 +1 @@
+Subproject commit 9a0ee2542848ab8625984fc8cdbfb9b5414c0082

llarp/CMakeLists.txt

@@ -15,6 +15,7 @@ set(LIB_UTIL_SRC
   util/logging/file_logger.cpp
   util/logging/json_logger.cpp
   util/logging/logger.cpp
+  util/logging/logger_internal.cpp
   util/logging/loglevel.cpp
   util/logging/ostream_logger.cpp
   util/logging/syslog_logger.cpp
@@ -42,10 +43,11 @@ endif()
 
 target_link_libraries(${UTIL_LIB} PUBLIC ${CRYPTOGRAPHY_LIB} ${LOG_LIB} ${CURL_LIBRARIES})
 target_link_libraries(${UTIL_LIB} PUBLIC
-    absl::time absl::hash
+    absl::hash
     nlohmann_json::nlohmann_json
     ghc_filesystem
     optional-lite
+    date::date
     )
 
 # cut back on fluff

llarp/util/logging/logger_internal.cpp

@@ -0,0 +1,15 @@
+#include <util/logging/logger_internal.hpp>
+
+#include <date/date.h>
+
+namespace llarp
+{
+  std::ostream&
+  operator<<(std::ostream& out, const log_timestamp& ts)
+  {
+    std::chrono::time_point< std::chrono::system_clock,
+                             std::chrono::milliseconds >
+        now{std::chrono::milliseconds{ts.now}};
+    return date::operator<<(out, now) << " UTC [+" << ts.delta << " ms]";
+  }
+}  // namespace llarp

llarp/util/logging/logger_internal.hpp

@@ -3,28 +3,23 @@
 
 #include <util/time.hpp>
 
-#include <absl/time/clock.h>
-#include <absl/time/time.h>
 #include <ctime>
-#include <iomanip>
 #include <sstream>
 #include <util/thread/threading.hpp>
 
 namespace llarp
 {
-  /** internal */
-  template < typename TArg >
-  void
-  LogAppend(std::stringstream& ss, TArg&& arg) noexcept
+  /** internal, recursion terminator */
+  constexpr void
+  LogAppend(std::stringstream&) noexcept
   {
-    ss << std::forward< TArg >(arg);
   }
   /** internal */
   template < typename TArg, typename... TArgs >
   void
   LogAppend(std::stringstream& ss, TArg&& arg, TArgs&&... args) noexcept
   {
-    LogAppend(ss, std::forward< TArg >(arg));
+    ss << std::forward< TArg >(arg);
     LogAppend(ss, std::forward< TArgs >(args)...);
   }
 
@@ -54,18 +49,8 @@ namespace llarp
     explicit log_timestamp(const char* fmt);
   };
 
-  inline std::ostream&
-  operator<<(std::ostream& out, const log_timestamp& ts)
-  {
-#if defined(ANDROID) || defined(RPI)
-    (void)ts;
-    return out << ts.now << " [+" << ts.delta << " ms]";
-#else
-    absl::TimeZone tz = absl::LocalTimeZone();
-    return out << absl::FormatTime(ts.format, absl::FromUnixMillis(ts.now), tz)
-               << " [+" << ts.delta << " ms]";
-#endif
-  }
+  std::ostream&
+  operator<<(std::ostream& out, const log_timestamp& ts);
 
 }  // namespace llarp