无法正确格式化日期时间变量

Question

我试图将日期时间转换为某种格式：

Timex.format!(my_date_time, "%a, %d %b %Y %H:%M:%S %z"))

抛出异常：

 got {:format, "Invalid format string, must contain at least one directive."} 
 while retrieving Exception.message/1 for %Timex.Format.FormatError{message: 
{:format, "Invalid format string, must contain at least one directive."}} (expected a string)