# 12-Hour Time with AM/PM Key: `datetime.time.hms_12h` 12-hour time with AM/PM indicator. Standard in US and some Commonwealth countries. Page: Format: `%I:%M:%S %p` Samples: `10:30:00 AM`, `11:59:59 PM`, `12:00:00 AM` ## Background ### Origin The 12-hour clock divides the 24 hours of the day into two periods, a.m. and p.m.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) The abbreviation a.m. comes from the Latin ante meridiem, meaning before midday, and p.m. from post meridiem, meaning after midday.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) The 12-hour clock can be traced back as far as Mesopotamia and ancient Egypt.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) The 12-hour clock is the dominant written and spoken system of time in several countries, predominantly nations that were part of the former British Empire.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) The United States, Canada outside Quebec, Australia, India and the Philippines are among the countries Wikipedia lists as following the 12-hour convention.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) In most countries the 24-hour clock is the standard system, especially in writing.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) ### Use NIST's Times of Day FAQs say that 12 a.m. and 12 p.m. are ambiguous and should not be used.[[2]](https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs) NIST explains that noon is neither before noon nor after noon, so neither a.m. nor p.m. is a correct designation for it.[[2]](https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs) By convention, 12 a.m. denotes midnight and 12 p.m. denotes noon.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) Historically, the style manual of the United States Government Printing Office used 12 a.m. for noon and 12 p.m. for midnight, though this was reversed in its 2008 editions.[[1]](https://en.wikipedia.org/wiki/12-hour_clock) When a column holds 12:00:00 AM or 12:00:00 PM, check how the source system meant them before trusting a midnight or a noon, especially in older records. (Meridian's view) In Python's strptime, the %p directive only affects the parsed hour if the %I directive is used to parse the hour.[[3]](https://docs.python.org/3/library/datetime.html) DuckDB's strptime function throws an error when a value does not match the format.[[4]](https://duckdb.org/docs/current/sql/functions/dateformat) DuckDB's try\_strptime function returns NULL on failure instead of raising an error.[[4]](https://duckdb.org/docs/current/sql/functions/dateformat) Parsing with a function that returns NULL on failure, then counting the NULLs, is a quick way to find the rows that do not fit before you cast the whole column. (Meridian's view) Python's documentation gives AM and PM as the en\_US values of %p, and am and pm as the de\_DE values.[[3]](https://docs.python.org/3/library/datetime.html) The Unicode LDML standard says the AM and PM day period may be upper or lowercase depending on the locale and other options.[[5]](https://www.unicode.org/reports/tr35/tr35-dates.html) Normalise the case of the marker, and strip any dots from forms like a.m., before parsing a column gathered from more than one system. (Meridian's view) In the Unicode LDML date patterns, the symbol h is the hour from 1 to 12 written with the minimum number of digits, while hh is zero-padded to two digits.[[5]](https://www.unicode.org/reports/tr35/tr35-dates.html) CLDR's English data gives the medium time format, in its ASCII variant, as h:mm:ss a.[[6]](https://raw.githubusercontent.com/unicode-org/cldr-json/main/cldr-json/cldr-dates-full/main/en/ca-gregorian.json) Unicode's CLDR time patterns have used a narrow no-break space between the time and AM/PM since CLDR 42.[[7]](https://blog.unicode.org/2023/06/icu-732-cldr-431-released-gb18030.html) CLDR includes optional variants of the English time formats with AM/PM that use ASCII spaces.[[7]](https://blog.unicode.org/2023/06/icu-732-cldr-431-released-gb18030.html) JDK 20 upgraded its locale data to CLDR version 42, which replaces regular spaces with no-break or narrow no-break spaces in places including time formats between the a field and the time.[[8]](https://inside.java/2024/03/29/quality-heads-up/) Inside.java warns that after the change, code parsing a string built with a regular space can fail because the parser expects an NBSP or NNBSP.[[8]](https://inside.java/2024/03/29/quality-heads-up/) If a 12-hour time that looks right refuses to parse, look for an invisible U+202F or U+00A0 before the AM or PM, and replace it with a plain space first. (Meridian's view) ## Sources 1. [12-hour clock](https://en.wikipedia.org/wiki/12-hour_clock) — secondary source 2. [Times of Day FAQs](https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs) — documentation 3. [datetime — Basic date and time types](https://docs.python.org/3/library/datetime.html) — documentation 4. [Date Format Functions](https://duckdb.org/docs/current/sql/functions/dateformat) — documentation 5. [Unicode Locale Data Markup Language (LDML) Part 4: Dates](https://www.unicode.org/reports/tr35/tr35-dates.html) — standard 6. [ca-gregorian.json](https://raw.githubusercontent.com/unicode-org/cldr-json/main/cldr-json/cldr-dates-full/main/en/ca-gregorian.json) — standard 7. [The Unicode Blog: ICU 73.2 \& CLDR 43.1 released: GB18030 compliance updates \& compatibility fixes](https://blog.unicode.org/2023/06/icu-732-cldr-431-released-gb18030.html) — secondary source 8. [Quality Outreach Heads-up - JDK 20-23: Support for Unicode CLDR Version 42](https://inside.java/2024/03/29/quality-heads-up/) — documentation