The format string consists of zero or more conversion specifiers, whitespace characters, and ordinary characters (except %). Each ordinary character is expected to match one character in the input stream in case-insensitive comparison. Each whitespace character matches arbitrary whitespace in the input string. Each conversion specification begins with % character, optionally followed by E or O modifier (ignored if unsupported by the locale), followed by the character that determines the behavior of the specifier. The format specifiers match the POSIX function strptime():
| Conversion specifier
 | Explanation | Writes to fields | 
| % | matches a literal %. The full conversion specification must be %%. | (none) | 
| t | matches any whitespace. | (none) | 
| n | matches any whitespace. | (none) | 
| Year | 
|---|
| Y | parses full year as a 4 digit decimal number, leading zeroes permitted but not required | tm_year | 
| EY | parses year in the alternative representation, e.g.平成23年 (year Heisei 23) which writes 2011 to tm_year in ja_JP locale | tm_year | 
| y | parses last 2 digits of year as a decimal number. Range [69,99] results in values 1969 to 1999, range [00,68] results in 2000-2068 | tm_year | 
| Oy | parses last 2 digits of year using the alternative numeric system, e.g. 十一 is parsed as 11 in ja_JP locale | tm_year | 
| Ey | parses year as offset from locale's alternative calendar period %EC | tm_year | 
| C | parses the first 2 digits of year as a decimal number (range [00,99]) | tm_year | 
| EC | parses the name of the base year (period) in the locale's alternative representation, e.g. 平成 (Heisei era) in ja_JP | tm_year | 
| Month | 
|---|
| b | parses the month name, either full or abbreviated, e.g. Oct | tm_mon | 
| h | synonym of b | tm_mon | 
| B | synonym of b | tm_mon | 
| m | parses the month as a decimal number (range [01,12]), leading zeroes permitted but not required | tm_mon | 
| Om | parses the month using the alternative numeric system, e.g. 十二 parses as 12 in ja_JP locale | tm_mon | 
| Week | 
|---|
| U | parses the week of the year as a decimal number (Sunday is the first day of the week) (range [00,53]), leading zeroes permitted but not required | tm_year, tm_wday, tm_yday | 
| OU | parses the week of the year, as by %U, using the alternative numeric system, e.g. 五十二 parses as 52 in ja_JP locale | tm_year, tm_wday, tm_yday | 
| W | parses the week of the year as a decimal number (Monday is the first day of the week) (range [00,53]), leading zeroes permitted but not required | tm_year, tm_wday, tm_yday | 
| OW | parses the week of the year, as by %W, using the alternative numeric system, e.g. 五十二 parses as 52 in ja_JP locale | tm_year, tm_wday, tm_yday | 
| Day of the year/month | 
|---|
| j | parses day of the year as a decimal number (range [001,366]), leading zeroes permitted but not required | tm_yday | 
| d | parses the day of the month as a decimal number (range [01,31]), leading zeroes permitted but not required | tm_mday | 
| Od | parses the day of the month using the alternative numeric system, e.g 二十七 parses as 23 in ja_JP locale, leading zeroes permitted but not required | tm_mday | 
| e | synonym of d | tm_mday | 
| Oe | synonym of Od | tm_mday | 
| Day of the week | 
|---|
| a | parses the name of the day of the week, either full or abbreviated, e.g. Fri | tm_wday | 
| A | synonym of a | tm_wday | 
| w | parses weekday as a decimal number, where Sunday is 0 (range [0-6]) | tm_wday | 
| Ow | parses weekday as a decimal number, where Sunday is 0, using the alternative numeric system, e.g. 二 parses as 2 in ja_JP locale | tm_wday | 
| Hour, minute, second | 
|---|
| H | parses the hour as a decimal number, 24 hour clock (range [00-23]), leading zeroes permitted but not required | tm_hour | 
| OH | parses hour from 24-hour clock using the alternative numeric system, e.g. 十八 parses as 18 in ja_JP locale | tm_hour | 
| I | parses hour as a decimal number, 12 hour clock (range [01,12]), leading zeroes permitted but not required | tm_hour | 
| OI | parses hour from 12-hour clock using the alternative numeric system, e.g. 六 reads as 06 in ja_JP locale | tm_hour | 
| M | parses minute as a decimal number (range [00,59]), leading zeroes permitted but not required | tm_min | 
| OM | parses minute using the alternative numeric system, e.g. 二十五 parses as 25 in ja_JP locale | tm_min | 
| S | parses second as a decimal number (range [00,60]), leading zeroes permitted but not required | tm_sec | 
| OS | parses second using the alternative numeric system, e.g. 二十四 parses as 24 in ja_JP locale | tm_sec | 
| Other | 
|---|
| c | parses the locale's standard date and time string format, e.g. Sun Oct 17 04:41:13 2010 (locale dependent) | all | 
| Ec | parses the locale's alternative date and time string format, e.g. expecting 平成23年 (year Heisei 23) instead of 2011年 (year 2011) in ja_JP locale | all | 
| x | parses the locale's standard date representation | all | 
| Ex | parses the locale's alternative date representation, e.g. expecting 平成23年 (year Heisei 23) instead of 2011年 (year 2011) in ja_JP locale | all | 
| X | parses the locale's standard time representation | all | 
| EX | parses the locale's alternative time representation | all | 
| D | equivalent to "%m / %d / %y " | tm_mon, tm_mday, tm_year | 
| r | parses locale's standard 12-hour clock time (in POSIX, "%I : %M : %S %p") | tm_hour, tm_min, tm_sec | 
| R | equivalent to "%H : %M" | tm_hour, tm_min | 
| T | equivalent to "%H : %M : %S" | tm_hour, tm_min, tm_sec | 
| p | parses the locale's equivalent of a.m. or p.m. | tm_hour |