diff --git a/libcron/include/libcron/CronData.h b/libcron/include/libcron/CronData.h
index c402517..94ede9b 100644
--- a/libcron/include/libcron/CronData.h
+++ b/libcron/include/libcron/CronData.h
@@ -142,13 +142,13 @@ namespace libcron
         for (const auto& name : names)
         {
             std::regex m(name, std::regex_constants::ECMAScript | std::regex_constants::icase);
-            for (size_t i = 0; i < parts.size(); ++i)
+            for (auto& part : parts)
             {
                 std::string replaced;
-                std::regex_replace(std::back_inserter(replaced), parts[i].begin(), parts[i].end(), m,
+                std::regex_replace(std::back_inserter(replaced), part.begin(), part.end(), m,
                                    std::to_string(value_of_first_name));
 
-                parts[i] = replaced;
+                part = replaced;
             }
 
             value_of_first_name++;
@@ -235,8 +235,8 @@ namespace libcron
 
         if (std::regex_match(s.begin(), s.end(), match, range))
         {
-            auto left = std::stoi(match[1].str().c_str());
-            auto right = std::stoi(match[2].str().c_str());
+            auto left = std::stoi(match[1].str());
+            auto right = std::stoi(match[2].str());
 
             if (is_within_limits<T>(left, right))
             {
@@ -269,10 +269,10 @@ namespace libcron
             }
             else
             {
-                raw_start = std::stoi(match[1].str().c_str());
+                raw_start = std::stoi(match[1].str());
             }
 
-            auto raw_step = std::stoi(match[2].str().c_str());
+            auto raw_step = std::stoi(match[2].str());
 
             if (is_within_limits<T>(raw_start, raw_start) && raw_step > 0)
             {
diff --git a/libcron/src/CronData.cpp b/libcron/src/CronData.cpp
index 6c08f59..dcfb902 100644
--- a/libcron/src/CronData.cpp
+++ b/libcron/src/CronData.cpp
@@ -90,15 +90,7 @@ namespace libcron
             // Make sure that if the days contains only 31, at least one month allows that date.
             if (day_of_month.size() == 1 && day_of_month.find(DayOfMonth::Last) != day_of_month.end())
             {
-                std::vector<int32_t> months_with_31;
-                for (int32_t i = 1; i <= 12; ++i)
-                {
-                    auto ymd = 2018_y / i / date::last;
-                    if (unsigned(ymd.day()) == 31)
-                    {
-                        months_with_31.push_back(i);
-                    }
-                }
+                constexpr std::array<uint32_t, 7> months_with_31{1, 3, 5, 7, 8, 10, 12};
 
                 res = false;
                 for (size_t i = 0; !res && i < months_with_31.size(); ++i)