use static constexpr

Author: FujiwaraNaoto

trantor/utils/Date.h

@@ -18,8 +18,6 @@
 #include <stdint.h>
 #include <string>
 
-#define MICRO_SECONDS_PER_SEC 1000000LL
-
 namespace trantor
 {
 /**
@@ -347,6 +345,8 @@ class TRANTOR_EXPORT Date
         std::swap(microSecondsSinceEpoch_, that.microSecondsSinceEpoch_);
     }
 
+    static constexpr long MICRO_SECONDS_PER_SEC = 1000000LL;
+
   private:
     int64_t microSecondsSinceEpoch_{0};
 };