diff --git a/.clang-tidy b/.clang-tidy
new file mode 100644
index 0000000..56b09c7
--- /dev/null
+++ b/.clang-tidy
@@ -0,0 +1,26 @@
+---
+Checks: "-*,\
+boost-*,\
+cert-*,\
+clang-analyzer-*,\
+cppcoreguidelines-*,\
+-cppcoreguidelines-macro-usage,\
+-cppcoreguidelines-owning-memory,\
+-hicpp-*,\
+misc-*,\
+-misc-non-private-member-variables-in-classes,\
+-modernize-*,\
+-modernize-use-trailing-return-type,\
+performance-*,\
+portability-*,\
+readability-*,\
+-*magic-numbers,\
+-*avoid-c-arrays,\
+"
+WarningsAsErrors: ''
+HeaderFilterRegex: '.*'
+AnalyzeTemporaryDtors: false
+FormatStyle:     file
+User:            clausklein
+...
+
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2e0e3ae..cde9bf8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.14...3.19)
 
 # ---- Project ----
 
@@ -21,15 +21,16 @@ endif()
 # ---- Project settings ----
 
 if(NOT DEFINED CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD 20)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
   set(CMAKE_CXX_EXTENSIONS NO)
 endif()
 
 # ---- Add dependencies via CPM ----
-# see https://github.com/TheLartians/CPM.cmake for more info
+# see https://github.com/cpm-cmake/CPM.cmake for more info
 
 include(cmake/CPM.cmake)
+cpmusepackagelock(package-lock.cmake)
 
 # PackageProject.cmake will be used to make our target installable
 CPMAddPackage(
@@ -59,8 +60,9 @@ file(GLOB_RECURSE sources CONFIGURE_DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/source/
 
 # Note: for header-only libraries change all PUBLIC flags to INTERFACE and create an interface
 # target! EITHER: add_library(Greeter INTERFACE) OR:
-add_library(Greeter ${headers} ${sources})
+add_library(Greeter SHARED ${headers} ${sources})
 
+# EITHER:
 set_target_properties(Greeter PROPERTIES CXX_STANDARD ${CMAKE_CXX_STANDARD})
 # OR target_compile_features(Greeter PUBLIC cxx_std_17)
 
@@ -69,9 +71,9 @@ if(MSVC)
   target_compile_options(Greeter PUBLIC /permissive)
 endif()
 
-# Link dependencies (if required)
+# Link dependencies EITHER:
 target_link_libraries(Greeter PRIVATE $<BUILD_INTERFACE:fmt::fmt-header-only>)
-# XXX target_link_libraries(Greeter PUBLIC fmt::fmt)
+# OR: target_link_libraries(Greeter PUBLIC fmt::fmt)
 
 target_include_directories(
   Greeter PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
@@ -93,6 +95,6 @@ packageProject(
   INCLUDE_DIR ${PROJECT_SOURCE_DIR}/include
   INCLUDE_DESTINATION include/${PROJECT_NAME}-${PROJECT_VERSION}
   VERSION_HEADER "${VERSION_HEADER_LOCATION}"
-  # TODO COMPATIBILITY SameMajorVersion
-  DEPENDENCIES "fmt 7.1.3"
+  COMPATIBILITY SameMajorVersion
+  # NOTE: not needed DEPENDENCIES "fmt 7.1.3"
 )
diff --git a/GNUmakefile b/GNUmakefile
new file mode 100755
index 0000000..a177583
--- /dev/null
+++ b/GNUmakefile
@@ -0,0 +1,58 @@
+#
+# CURDIR=$(/bin/pwd)
+#
+# GENERATOR="Unix Makefiles"
+GENERATOR=Ninja
+
+export CPM_SOURCE_CACHE=${HOME}/.cache/CPM
+export CPM_USE_LOCAL_PACKAGES=1
+
+.PHONY: update format all test check clean distclean lock
+
+# the default target does just all
+check: all
+
+all: test
+
+test: install
+
+
+distclean:
+	rm -rf build root
+
+# update CPM.cmake
+update:
+	wget -q -O cmake/CPM.cmake https://github.com/cpm-cmake/CPM.cmake/releases/latest/download/get_cpm.cmake
+
+lock: install
+	cmake --build build/install --target cpm-update-package-lock
+
+# install the library
+install:
+	cmake -S . -B build/install -G "${GENERATOR}" -DCMAKE_PREFIX_PATH=${CURDIR}/root -DCMAKE_INSTALL_PREFIX=${CURDIR}/root -DCMAKE_CXX_STANDARD=20 # --trace-expand
+	cmake --build build/install --target install
+
+# test the library
+test:
+	cmake -S test -B build/test -G "${GENERATOR}" -DCMAKE_PREFIX_PATH=${CURDIR}/root -DTEST_INSTALLED_VERSION=1
+	cmake --build build/test
+	cmake --build build/test --target test
+
+# all together
+all:
+	cmake -S all -B build/all -G "${GENERATOR}" -DCMAKE_PREFIX_PATH=${CURDIR}/root -DTEST_INSTALLED_VERSION=1 -DENABLE_TEST_COVERAGE=1
+	cmake --build build/all
+	cmake --build build/all --target test
+	cmake --build build/all --target GenerateDocs
+	cmake --build build/all --target check-format
+
+format: distclean
+	find . -name CMakeLists.txt | xargs cmake-format -i
+	find . -name '*.cpp' | xargs clang-format -i
+	find . -name '*.h' | xargs clang-format -i
+
+# check the library
+check:
+	cmake -S standalone -B build/standalone -G "${GENERATOR}" -DCMAKE_PREFIX_PATH=${CURDIR}/root -DCMAKE_EXPORT_COMPILE_COMMANDS=1
+	cmake --build build/standalone --target all
+	run-clang-tidy.py -p build/standalone -checks='-*,modernize-*,misc-*,hicpp-*,cert-*,readability-*,portability-*,performance-*,google-*' standalone
diff --git a/documentation/CMakeLists.txt b/documentation/CMakeLists.txt
index 8f5cbfe..b3717dc 100644
--- a/documentation/CMakeLists.txt
+++ b/documentation/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.14...3.19)
 
 project(GreeterDocs)
 
diff --git a/include/greeter/greeter.h b/include/greeter/greeter.h
index 9d367d7..19569eb 100644
--- a/include/greeter/greeter.h
+++ b/include/greeter/greeter.h
@@ -18,17 +18,14 @@ namespace greeter {
      * @brief Creates a new greeter
      * @param name the name to greet
      */
-    Greeter(std::string name);
+    explicit Greeter(std::string name);
 
     /**
      * @brief Creates a localized string containing the greeting
      * @param lang the language to greet in
      * @return a string containing the greeting
      */
-    std::string greet(LanguageCode lang = LanguageCode::EN) const;
-
-    /// @brief Return an iso date string
-    std::string getIsoDate() const;
+    [[nodiscard]] std::string greet(LanguageCode lang = LanguageCode::EN) const;
   };
 
 }  // namespace greeter
diff --git a/source/greeter.cpp b/source/greeter.cpp
index 1a5bfb8..11fb416 100644
--- a/source/greeter.cpp
+++ b/source/greeter.cpp
@@ -1,4 +1,4 @@
-#include <fmt/chrono.h>
+#include <fmt/format.h>
 #include <greeter/greeter.h>
 
 using namespace greeter;
@@ -9,17 +9,12 @@ std::string Greeter::greet(LanguageCode lang) const {
   switch (lang) {
     default:
     case LanguageCode::EN:
-      return "Hello, " + name + "!";
+      return fmt::format("Hello, {}!", name);
     case LanguageCode::DE:
-      return "Hallo " + name + "!";
+      return fmt::format("Hallo {}!", name);
     case LanguageCode::ES:
-      return "¡Hola " + name + "!";
+      return fmt::format("¡Hola {}!", name);
     case LanguageCode::FR:
-      return "Bonjour " + name + "!";
+      return fmt::format("Bonjour {}!", name);
   }
 }
-
-std::string Greeter::getIsoDate() const {
-  using namespace std::literals::chrono_literals;
-  return fmt::format("{:%H:%M:%S}", 3h + 15min + 30s);
-}
diff --git a/standalone/CMakeLists.txt b/standalone/CMakeLists.txt
index 4bb837d..bcf0be6 100644
--- a/standalone/CMakeLists.txt
+++ b/standalone/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.14...3.19)
 
 project(GreeterStandalone LANGUAGES CXX)
 
@@ -27,4 +27,8 @@ add_executable(GreeterStandalone ${sources})
 
 set_target_properties(GreeterStandalone PROPERTIES CXX_STANDARD 17 OUTPUT_NAME "Greeter")
 
-target_link_libraries(GreeterStandalone Greeter::Greeter cxxopts)
+# WORKAROUND for ALIAS target is missing error! CK
+if(NOT TARGET cxxopts::cxxopts)
+  add_library(cxxopts::cxxopts ALIAS cxxopts)
+endif()
+target_link_libraries(GreeterStandalone Greeter::Greeter cxxopts::cxxopts)
diff --git a/standalone/source/main.cpp b/standalone/source/main.cpp
index 5464fa5..6874460 100644
--- a/standalone/source/main.cpp
+++ b/standalone/source/main.cpp
@@ -6,15 +6,20 @@
 #include <string>
 #include <unordered_map>
 
-const std::unordered_map<std::string, greeter::LanguageCode> languages{
-    {"en", greeter::LanguageCode::EN},
-    {"de", greeter::LanguageCode::DE},
-    {"es", greeter::LanguageCode::ES},
-    {"fr", greeter::LanguageCode::FR},
-};
-
+// NOLINTNEXTLINE(modernize-use-trailing-return-type)
 int main(int argc, char** argv) {
-  cxxopts::Options options(argv[0], "A program to welcome the world!");
+  // prevent warning: initialization of 'languages' with static storage duration may throw an
+  // exception that cannot be caught [cert-err58-cpp]
+  const std::unordered_map<std::string, greeter::LanguageCode> languages{
+      {"en", greeter::LanguageCode::EN},
+      {"de", greeter::LanguageCode::DE},
+      {"es", greeter::LanguageCode::ES},
+      {"fr", greeter::LanguageCode::FR},
+  };
+
+  // prevent warning: do not use pointer arithmetic
+  // [cppcoreguidelines-pro-bounds-pointer-arithmetic]
+  cxxopts::Options options(*argv, "A program to welcome the world!");
 
   std::string language;
   std::string name;
@@ -33,7 +38,10 @@ int main(int argc, char** argv) {
   if (result["help"].as<bool>()) {
     std::cout << options.help() << std::endl;
     return 0;
-  } else if (result["version"].as<bool>()) {
+  }
+
+  // prevent warning: do not use 'else' after 'return' [readability-else-after-return]
+  if (result["version"].as<bool>()) {
     std::cout << "Greeter, version " << GREETER_VERSION << std::endl;
     return 0;
   }
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 76def2c..5fa8612 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.14...3.19)
 
 project(GreeterTests LANGUAGES CXX)
 
@@ -57,18 +57,11 @@ endif()
 
 enable_testing()
 
-# Note: doctest and similar testing frameworks can automatically configure CMake tests For other
-# testing frameworks add the tests target instead:
-if(DEFINED doctest_SOURCE_DIR)
-  include(${doctest_SOURCE_DIR}/scripts/cmake/doctest.cmake)
-  doctest_discover_tests(GreeterTests)
-else()
-  add_test(greeterTests GreeterTests)
-endif()
+add_test(greeterTests GreeterTests)
 
 # ---- code coverage ----
 
-if(ENABLE_TEST_COVERAGE)
+if(ENABLE_TEST_COVERAGE AND NOT Greeter_FOUND)
   target_compile_options(Greeter PUBLIC -O0 -g -fprofile-arcs -ftest-coverage)
   target_link_options(Greeter PUBLIC -fprofile-arcs -ftest-coverage)
 endif()
diff --git a/test/source/greeter.cpp b/test/source/greeter.cpp
index cab989b..10a1242 100644
--- a/test/source/greeter.cpp
+++ b/test/source/greeter.cpp
@@ -24,8 +24,3 @@ TEST_CASE("Greeter version") {
   CHECK(std::string(GREETER_VERSION) == std::string("1.0"));
 #endif
 }
-
-TEST_CASE("Greeter date") {
-  const greeter::Greeter greeter("Tests");
-  CHECK(greeter.getIsoDate() == std::string("03:15:30"));
-}