add secondsSinceEpoch to trantor::Date (#117)

Author: kotori2

trantor/utils/Date.h

@@ -167,6 +167,16 @@ class Date
         return microSecondsSinceEpoch_;
     }
 
+    /**
+     * @brief Get the number of seconds since 1970-01-01 00:00.
+     *
+     * @return int64_t
+     */
+    int64_t secondsSinceEpoch() const
+    {
+        return microSecondsSinceEpoch_ / MICRO_SECONDS_PRE_SEC;
+    }
+
     /**
      * @brief Get the tm struct for the time point.
      *