Revert "Use a different Date data type"

This reverts commit 857c7d3972.

src/utils/datetime.hpp

@@ -1,56 +0,0 @@
-#pragma once
-
-#include <chrono>
-#include <string>
-
-#include <logger/logger.hpp>
-
-class DateTime
-{
-public:
-  enum class Engine {
-    Postgresql,
-    Sqlite3,
-  } engine{Engine::Sqlite3};
-
-  using time_point = std::chrono::system_clock::time_point;
-
-  DateTime():
-      s{},
-      t{}
-  { }
-
-  DateTime(std::time_t t):
-      t(std::chrono::seconds(t))
-  {}
-
-  DateTime(std::string s):
-      s(std::move(s))
-  {}
-
-  DateTime& operator=(const std::string& s)
-  {
-    this->s = s;
-    return *this;
-  }
-
-  DateTime& operator=(const time_point t)
-  {
-    this->t = t;
-    return *this;
-  }
-
-  const std::string& to_string() const
-  {
-    return this->s;
-  }
-
-  time_point::duration epoch() const
-  {
-    return this->t.time_since_epoch();
-  }
-
-private:
-  std::string s;
-  time_point t;
-};