calendar - Erlang 的时间日期格式处理转换函数库
本地时间和国际标准时间（UTC: Universal Coordinated Time），以及星期格式、日期和时间分秒的处理转换
- date_to_gregorian_days/1
- date_to_gregorian_days/3
- datetime_to_gregorian_seconds/1
- day_of_the_week/1
- day_of_the_week/3
- gregorian_days_to_date/1
- gregorian_seconds_to_datetime/1
- is_leap_year/1
- iso_week_number/0
- iso_week_number/1
- last_day_of_the_month/2
- local_time/0
- local_time_to_universal_time/1
- local_time_to_universal_time/2
- local_time_to_universal_time_dst/1
- now_to_datetime/1
- now_to_local_time/1
- now_to_universal_time/1
- seconds_to_daystime/1
- seconds_to_time/1
- time_difference/2
- time_to_seconds/1
- universal_time/0
- universal_time_to_local_time/1
- valid_date/1
- valid_date/3