A Time Library for Dylan

Current status as of Oct 2021:

• Most APIs work, but on Linux only.
• No TZ data yet so only naive time zones work.
• Needs more tests.
• No monotonic clock support yet.

This library is intended to be an improvement on the Open Dylan date library for several reasons:

• More complete -- it will have full time zone support.
• More efficient -- each <time> instance uses far less memory.
• More precision -- nanoseconds rather than microseconds.
• Better naming -- What can I say? I just never liked the name <date> to represent an instant in time.

The development of this library was influenced to some extent by the time libraries in Go, Common Lisp (local-time), Python, and Rust, in about that order. I used them as a reference to make sure I wasn't doing anything too crazy.

High Level Overview

NOTE: The first version of this library assumes 64-bit integers. The contortions necessary to make it work on 32-bit were more than I wanted to deal with. My plan is to make that work by using <double-integer> on 32-bit architectures, but I don't want to pay the price in speed for doing that on the more common 64-bit platforms.

This section contains a brief overview of the time library to help you get a quick feel for how it is organized. See the reference documentation for more detail.

The time library exports a single module, time which exports these classes:

• <time> - an instant in time, an offset from the Unix Epoch. Has an associated time zone for use when converting to the time in a specific location on Earth.

• <duration> - the elapsed time between two time instants, to nanosecond precision.

• <zone> - A time zone, either "naive" or "aware". (See the next two classes.)

• <naive-zone> - A time zone that always represents the same offset from UTC. The primary example of this is the $utc time zone itself.

• <aware-zone> - A time zone that may have different abbreviations and offsets from UTC over time due to daylight savings time and governmental action. These zones use data from the the tzinfo (TODO: link) package.

• <day> - Represents a day of the week, with a name and short name.

• <month> - Represents a month, January to December, with a name, short name, and number from 1 to 12.

The library API can be logically separated into several parts:

• Constructors - <time> objects are usually created by calling time-now, parse-time, or by constructing one from components with compose-time(year, month, day, ...). It is also fine to call make(<time>) directly.

• Accessors - for example to extract the number of seconds from a time or the number of days in a month.

• Conversions - for converting times and durations to and from strings, composing/decomposing them from/into their parts, or converting to a different zone.

• Comparisons - the =, <, and > functions work on pairs of times and pairs of durations.

• Arithmetic - the Dylan arithmetic operators are overloaded to work on combinations of times and durations for which it makes logical sense:

  • time + duration or duration + time
  • time - duration
  • duration + duration
  • duration - duration
  • duration * real number or real number * duration
  • duration / real number

Code Organization

• For each file named foo.dylan there is a corresponding file named foo-test.dylan containing unit tests. Unit tests are mainly written for the public interfaces, but a few internals are exported from the %time module to enable verification of results.

• For each exported class I try to keep the methods for initialization (make, initialize) immediately after the class definition. Then the generic functions that apply only to that class. Then implementation methods.

TODO

• Platform other than x86_64-linux. Do we want to support 32-bit?

• Like Rust's time::Instant, uses monotonic clock: define class <instant> (<abstract-time>) Is this needed?

• <date>, <naive-date>, <time-of-day>?

• Decide how/whether to deal with monotonic clocks here.

• Leap seconds. https://docs.rs/chrono/*/chrono/naive/struct.NaiveTime.html#leap-second-handling

• Calender operations? Could go in separate module or even library.

• i18n - ensure that if someone wanted to they could make the days, months, and date formats display/parse in non-English languages.

• The JavaScript Luxon library has some good API ideas. https://moment.github.io/luxon/#/tour In particular it would be nice to support intervals (time ranges), and the ISO duration format etc. It also has a nice discussion of time zones and some do's and don'ts.

References

• RFC2822, Internet Message Format, Date and Time Specification
• RFC5322, Internet Message Format, Date and Time Specification
• RFC3339, Date and Time on the Internet: Timestamps
• chrono-Compatible Low-Level Date Algorithms
• Discussion on julia-dev group is very enlightening about the distinctions between various time concepts.
• This blog post on some of the oddities of date/time arithmetic.
• RFC 8536, TZif format
• Calendrical Calculations -- The Ultimate Edition -- by Reingold and Dershowitz
• https://stackoverflow.com/questions/11188621/how-can-i-convert-seconds-since-the-epoch-to-hours-minutes-seconds-in-java/11197532#11197532
• https://stackoverflow.com/questions/7960318/math-to-convert-seconds-since-1970-into-date-and-vice-versa
• https://naggum.no/lugm-time.html