diff --git a/include/spdlog/pattern_formatter-inl.h b/include/spdlog/pattern_formatter-inl.h
index 25aa505b..f6506279 100644
--- a/include/spdlog/pattern_formatter-inl.h
+++ b/include/spdlog/pattern_formatter-inl.h
@@ -1011,7 +1011,7 @@ SPDLOG_INLINE void pattern_formatter::set_pattern(std::string pattern) {
 
 SPDLOG_INLINE void pattern_formatter::need_localtime(bool need) { need_localtime_ = need; }
 
-SPDLOG_INLINE std::tm pattern_formatter::get_time_(const details::log_msg &msg) {
+SPDLOG_INLINE std::tm pattern_formatter::get_time_(const details::log_msg &msg) const {
     if (pattern_time_type_ == pattern_time_type::local) {
         return details::os::localtime(log_clock::to_time_t(msg.time));
     }
diff --git a/include/spdlog/pattern_formatter.h b/include/spdlog/pattern_formatter.h
index 0658161c..42a7509e 100644
--- a/include/spdlog/pattern_formatter.h
+++ b/include/spdlog/pattern_formatter.h
@@ -99,7 +99,7 @@ private:
     std::vector<std::unique_ptr<details::flag_formatter>> formatters_;
     custom_flags custom_handlers_;
 
-    std::tm get_time_(const details::log_msg &msg);
+    std::tm get_time_(const details::log_msg &msg) const;
     template <typename Padder>
     void handle_flag_(char flag, details::padding_info padding);
 
diff --git a/include/spdlog/sinks/dup_filter_sink.h b/include/spdlog/sinks/dup_filter_sink.h
index 0588d772..1071e98b 100644
--- a/include/spdlog/sinks/dup_filter_sink.h
+++ b/include/spdlog/sinks/dup_filter_sink.h
@@ -78,8 +78,8 @@ protected:
     }
 
     // return whether the log msg should be displayed (true) or skipped (false)
-    bool filter_(const details::log_msg &msg) {
-        auto filter_duration = msg.time - last_msg_time_;
+    bool filter_(const details::log_msg &msg) const {
+        const auto filter_duration = msg.time - last_msg_time_;
         return (filter_duration > max_skip_duration_) || (msg.payload != last_msg_payload_);
     }
 };