zevv / janet-time

Introduction

Concept time/date/calendar library for the Janet programming language

The goal is to provide a lightweight library for working with times and dates. "complicated" functionaltiy like localization and time zone handling should be delagated to the operating system or C library when possible.

API

The functions that perform conversion between time and datetime types all have an optional time zone argument tz; this argument is a string with a RFC 2822/ISO 8601 time zone name.

when this argument is not given the functions act on the local time zone.

Read clock

The following functions all return time as a floating point number:

(time/now)

Read the real time clock. The result is a floating point number indicating the number of seconds since the UNIX epoch.

(time/monotonic)

Get current monotonic time. The result is a floating point number with an undefined epoch that is guarunteed to be monotonic.

(time/cputime)

Get CPU time used by current process. The result is a floating point number indicating the number of seconds the current process has spent executing on the CPU.

Conversion

(time/to-datetime time &opt tz):

Convert a time to a datetime struct.

(time/from-datetime dt &opt tz)

Convert a datetime struct to a time.

Formatting and parsing

(time/format time &opt fmt tz)

Format the given time to ASCII string. The format string is C89 strftime(3) format, or one of the predefined formats:

• :iso-8601
• :rfc-3339
• :rfc-2822
• :w3c

When the fmt argument is not given, it defaults to the :rfc-2822 format.

(time/parse fmt string &opt tz)

parse a time in the given format. The format string is a subset of the C89 strftime(3) format:

• %Y: year, 4 digits
• %y: year, 2 digits, 2000 is added if the year is less than 70
• %m: month, 2 digits, 01-12
• %d: day, 2 digits, 01-31
• %H: hour, 2 digits, 00-23
• %I: hour, 2 digits, 01-12
• %M: minute, 2 digits, 00-59
• %S: second, 2 digits, 00-59
• %p: AM/PM

or one of the predefined formats:

• :iso-8601
• :rfc-3339
• :rfc-2822
• :w3c

Design considerations and other ramblings

Basic concepts

• clock: some functiality measuring and providing time (typically provided by the OS)

• clock source: indentifier specifying a source of time: :monononic-time, :system-time, :cpu-time

• monotonic time: floating point representation of a monotonically increasing time in seconds with no defined epoch.

• system time: floating point representation of the system clock in seconds, relative to the system time epoch

• cpu time: floating point representation of the CPU time used by a process or thread.

• system time epoch: Represents the timestamp of the system time value 0: equal to the UNIX time epoch: January 1st 1970 00:00:00 UTC.

• interval: the difference between two monotime or system-time times, in seconds.

• time zone: a string identifying a world region time zone, for example "GMT", "UTC", "CET", "CET+3", etc. See ISO 8610.

• datetime: time components: year, month, day-of-month, day-of-year, weekday, hour, minute, second.

• UTC time: representation of a system time in datetime in Coordinated Universal Time.

• local time: representation of a system time in datetime in a given time zone.

• resolution: the smallest interval a given clock source can represent.

Functionality

The time library should offer the following functionality, properly handling time zones where appropiate:

• Querying system time, monotonic time and cpu time

• Breaking down system time to individual datetime components.

• Composing individual datetime components into a system time.

• Formatting datetime to human readable format using custom formatting or well-known time formats.

• Parsing datetime from human readable format using custom formatting or well-known time formats.

• Basic time arithmatic: adddition and subtraction of system time and datetime

Implementation

Clocks and sources

Querying time from a given clock source is provided by the stdlib os/clock and os/time functions. These are implemented with the following primitives:

• Windows: system time: GetSystemTimeAsFileTime(), monotonic time: QueryPerformanceCounter(), cpu time: GetProcessTimes()

• Apple: system time: host_get_clock_service(), monotonic time: host_get_clock_service(), cpu time: clock()

• Linux/POSIX: system time: clock_gettime(CLOCK_REALTIME), monotonic time: clock_gettime(CLOCK_MONOTONIC), cpu time: clock_gettime(CLOCK_PROCESS_CPUTIME_ID)

datetime

Janet already has a representation for broken down time as a date struct, https://janet-lang.org/api/index.html#os/date

Reuse this time? It uses a somewhat confusing 0-based indexing for day-of-month and month-of-year though.

Composing system time from datetime

Janet provides basic functionality through the C library mktime() and timegm() funcion wrapper os/mktime. mktime has no portable way for handling time zones though.

Splitting system time into datetime

Janet provides os/date as a wrapper around the C library localtime() and gmtime() functions. These have no portable way for handling time zones.

Localization

I don't even want to think about that. Delegate to C library

Time zones

See "Localization"

Time formatting

Some thoughts:

• Implementaion in native Janet janet should be easy for numerical times and dates, but will not support localazation for week and month names.

• Janet provides a wrapper around the C library strftime() function as os/strftime, which does take care of localzation.

• Time zone handling is not standardized for strftime()

Time parsing

POSIX defines the strptime() C function, which parses a struct tm from a given time format; this is not available on Windows.

Implementing time parsing in native Janet should not be too hard.