var timestamp = DateTime.UtcNow;

using (var command = new NpgsqlCommand())
{
  var sql = "INSERT INTO testtable (id, timestamp) VALUES (@id, @timestamp)";

  command.Connection = connection;
  command.CommandText = sql;
  command.Parameters.Add(new NpgsqlParameter("timestamp", timestamp));
  command.Parameters.Add(new NpgsqlParameter("id", 1));

  command.ExecuteNonQuery();
}

using (var command = new NpgsqlCommand())
  {
  var sql = "SELECT * FROM testtable WHERE id = @id";

  command.Connection = connection;
  command.CommandText = sql;
  command.Parameters.Add(new NpgsqlParameter("id", 1));

  var reader = command.ExecuteReader();
  if (reader.Read())
  {
    var resultTimestamp = Convert.ToDateTime(reader["timestamp"]);

    if (!Compare(resultTimestamp.ToUniversalTime(), timestamp.ToUniversalTime()))
      throw new Exception("Result was not as expected");
    }
  }

...

 static bool Compare(DateTime dt1, DateTime dt2)
 {
      return dt1.Year == dt2.Year &&
                 dt1.Month == dt2.Month &&
                 dt1.Day == dt2.Day &&
                 dt1.Hour == dt2.Hour &&
                 dt1.Minute == dt2.Minute &&
                 dt1.Second == dt2.Second;
 }