Most of the single letter literals were uppercase and that is not allowed by the standard and caused compilation error on GCC 7.3.0 (cygwin). Also we cannot just make all of them lowercase because of the month/minute conflict.

❯ bazel test ...
...
ERROR: /Users/yesudeep/code/bwuck/third_party/cc/test/BUILD.bazel:93:10: C++ compilation of rule '//third_party/cc/test:asap_example' failed (Exit 1): wrapped_clang failed: error executing command 
...
Use --sandbox_debug to see verbose messages from the sandbox wrapped_clang failed: error executing command 
  (cd /private/var/tmp/_bazel_yesudeep/148f9f6ebca6e47e7d6d5ed427a82e62/sandbox/darwin-sandbox/61/execroot/__main__ && \
  exec env - \
    APPLE_SDK_PLATFORM=MacOSX \
    APPLE_SDK_VERSION_OVERRIDE=11.0 \
    XCODE_VERSION_OVERRIDE=12.0.0.12A8189h \
  external/local_config_cc/wrapped_clang '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -O0 -DDEBUG '-std=c++11' -iquote . -iquote bazel-out/darwin-fastbuild/bin -iquote external/fmt -iquote bazel-out/darwin-fastbuild/bin/external/fmt -iquote external/mobius3_asap -iquote bazel-out/darwin-fastbuild/bin/external/mobius3_asap -iquote external/bazel_tools -iquote bazel-out/darwin-fastbuild/bin/external/bazel_tools -isystem external/fmt/include -isystem bazel-out/darwin-fastbuild/bin/external/fmt/include -isystem external/mobius3_asap/include -isystem bazel-out/darwin-fastbuild/bin/external/mobius3_asap/include -MD -MF bazel-out/darwin-fastbuild/bin/third_party/cc/test/_objs/asap_example/asap_example.d '-frandom-seed=bazel-out/darwin-fastbuild/bin/third_party/cc/test/_objs/asap_example/asap_example.o' -isysroot __BAZEL_XCODE_SDKROOT__ -F__BAZEL_XCODE_SDKROOT__/System/Library/Frameworks -F__BAZEL_XCODE_DEVELOPER_DIR__/Platforms/MacOSX.platform/Developer/Library/Frameworks '-mmacosx-version-min=11.0' -DHAVE_BAZEL_BUILD '-std=c++2a' -Wall -Wreturn-type -Wuninitialized -Wunused-result '-Werror=narrowing' '-Werror=reorder' -Wunused-local-typedefs '-Werror=conversion-null' '-Werror=overlength-strings' '-Werror=pointer-arith' '-Werror=varargs' '-Werror=vla' '-Werror=write-strings' -Wmissing-declarations '-fdiagnostics-color=always' -no-canonical-prefixes -Wno-builtin-macro-redefined '-D__DATE__="redacted"' '-D__TIMESTAMP__="redacted"' '-D__TIME__="redacted"' -c third_party/cc/test/asap_example.cc -o bazel-out/darwin-fastbuild/bin/third_party/cc/test/_objs/asap_example/asap_example.o)
Execution platform: @local_config_platform//:host

Use --sandbox_debug to see verbose messages from the sandbox
third_party/cc/test/asap_example.cc:11:15: error: use of overloaded operator '!=' is ambiguous (with operand types 'asap::detail::accessor<3600>::iterator' and 'asap::detail::accessor<3600>::iterator')
  for (auto d : period.every(asap::hour(2))) {
              ^
external/mobius3_asap/include/asap/period.tcc:49:16: note: candidate function
          bool operator!=(const iterator & other);
               ^
external/mobius3_asap/include/asap/period.tcc:48:16: note: candidate function
          bool operator==(const iterator & other);
               ^
external/mobius3_asap/include/asap/period.tcc:48:16: note: candidate function (with reversed parameter order)
third_party/cc/test/asap_example.cc:11:17: note: in implicit call to 'operator!=' for iterator of type 'asap::detail::accessor<3600>'
  for (auto d : period.every(asap::hour(2))) {
                ^~~~~~
external/mobius3_asap/include/asap/period.tcc:52:28: note: selected 'begin' function with iterator type 'accessor<3600>::iterator'
        accessor::iterator begin() const;
                           ^
1 error generated.
INFO: Elapsed time: 2.658s, Critical Path: 2.15s
INFO: 15 processes: 15 darwin-sandbox.
FAILED: Build did NOT complete successfully

Example code:

#include "asap/asap.h"
#include "fmt/core.h"
#include "fmt/format.h"

int
main() {
  auto now    = asap::now();
  auto period = now.until(asap::tomorrow());
  for (auto d : period.every(asap::hour(2))) {
    fmt::print("* {}\n", d.str());
  }
  return 0;
}