使用mktime函数，传入的结构体是struct tm。
要注意tm_year是计算的从1900年的值。
| tm_sec | int | seconds after the minute | 0-60* |
| tm_min | int | minutes after the hour | 0-59 |
| tm_hour | int | hours since midnight | 0-23 |
| tm_mday | int | day of the month | 1-31 |
| tm_mon | int | months since January | 0-11 |
| tm_year | int | years since 1900 | |
| tm_wday | int | days since Sunday | 0-6 |
| tm_yday | int | days since January 1 | 0-365 |
| tm_isdst | int | Daylight Saving Time flag |