localtime函数不是线程安全的，这个时间类可以完成获取当前时间，计算时间间隔，获取当前年月日等功能。该类可以在多线程中使用。-localtime function is not thread-safe, this time the class can be done to get the current time to calculate the time interval, to get the current date and other functions. This class can be use in a multi-threaded.