This list is pretty out of date at the moment. For a more complete list, trying Searching CPAN for "datetime".

The old list ...

• DateTime.pm (Dave Rolsky and others) - a base datetime object representing the Gregorian calendar. Also includes DateTime::Duration and DateTime::Infinite.
• DateTime::Locale (Richard Evans and Dave Rolsky) - modules for handling localization. Designed to work in conjunction with DateTime.pm.
• DateTime::TimeZone (Dave Rolsky) - modules for handling time zones. Designed to work in conjunction with DateTime.pm.
• DateTime::Set (Flavio Glock and Dave Rolsky) - a class for representing sets of datetime objects. Also includes DateTime::Span and DateTime::SpanSet.
• DateTime::Incomplete (Flavio Glock) - a class for representing incomplete datetime objects, where not all of the elements of the datetime are known.

• DateTime::Format::CLDR (Maroš Kollár) - parse and format CLDR time patterns
• DateTime::Format::Duration (Rick Measham and Karen Etheridge) - format and parse durations with strftime/strptime style patterns.
• DateTime::Format::Epoch (Eugene van der Pijll and Alexandr Ciornii) - translates DateTime objects to and from "seconds since the epoch", much more comprehensively than DateTime.pm itself does.
• DateTime::Format::Excel (Iain Truskett and Achim Bursian) - parses and formats Excel dates.
• DateTime::Format::HTTP (Iain Truskett and Christiaan Kras) - parses and formats dates uses the HTTP::Date module, which can parse a number of common datetime formats.
• DateTime::Format::ICal (Dave Rolsky) - parses and formats the iCal datetime and duration formats.
• DateTime::Format::ISO8601 (Joshua Hoblitt) - parses datetimes in various ISO8601 formats.
• DateTime::Format::Japanese (Daisuke Maki) - parses and formats datetime in Japanese.
• DateTime::Format::Mail (Iain Truskett and Philippe Bruhat (BooK)) - parses and formats RFC 2822 and RFC 822 (the email RFCs) style times.
• DateTime::Format::Natural (Steven Schubiger) - parses natural language dates, much like Date::Manip.
• DateTime::Format::Strptime (Rick Measham and Dave Rolsky) - implements strptime() for the DateTime project.
• DateTime::Format::W3CDTF (Kellan Elliot-McCrea and Gregory Todd Williams) - parses and formats the W3CDTG datetime strings.
• DateTime::Format::WindowsFileTime (Jim Garvin) - parses and formats the "Windows File Time" hex strings.

• DateTime::Format::DBI (Claus Färber) - given a DBI handle, picks the appropriate formatting module for the handle.
• DateTime::Format::MySQL (Dave Rolsky and Mike Wisener) - parses and formats dates in the formats output by MySQL.
• DateTime::Format::Pg (Claus Färber and Daisuke Maki) - parses and formats dates, times, and intervals in the formats output by Postgres.

• DateTime::Format::DateManip (Ben Bennett) - translates Date::Manip parseable strings to and from DateTime and DateTime::Duration objects.
• DateTime::Format::DateParse (Joshua Hoblitt) - translates Date::Parse parseable strings into DateTime objects.

• DateTime::Format::Baby (Rick Measham and Jean Forget, adapted from code by Abigail) - formats datetimes for babies.
• DateTime::Format::Bork (Joshua Hoblitt) - formats datetimes for that guy from The Muppet Show.
• DateTime::Format::IBeat (Iain Truskett) - parses and formats ".beat" times.
• DateTime::Format::Roman (Eugene van der Pijll, Dave Rolsky and Jean Forget) - formats dates using the Roman style. What have the Romans ever done for us?

• DateTime::Format::Builder (Iain Truskett and Dave Rolsky) - support code for creating DateTime::Format modules or one-off parser objects.

• DateTime::Calendar::Chinese (Daisuke Maki) - the Chinese calendar.
• DateTime::Calendar::Christian (Eugene van der Pijll and Tom Wyant) - the combined Julian/Gregorian calendar. The reform date is user-settable, so it is capable of representing the historical Western calendar for any location.
• DateTime::Calendar::Coptic (Daniel Yacob) - the Coptic calendar. Also includes DateTime::Calendar::CopticEthiopic.
• DateTime::Calendar::Discordian (Jaldhar H. Vyas) - the Discordian calendar.
• DateTime::Calendar::FrenchRevolutionary (Jean Forget) - the French Revolutionary calendar.
• DateTime::Calendar::Hebrew (Steven Weinberger and Raphael Mankin) - the Hebrew calendar.
• DateTime::Calendar::Hijri (Eugene van der Pijll) - a thin wrapper around Date::Hijri. Discussion on the datetime list suggests that this module is not very accurate. Future versions may a different algorithm.
• DateTime::Calendar::Japanese (Daisuke Maki) - the Japanese calendar.
• DateTime::Calendar::Julian (Eugene van der Pijll and Tom Wyant) - the Julian calendar.
• DateTime::Calendar::Mayan (Joshua Hoblitt) - the Mayan Long Count calendar.
• DateTime::Calendar::Pataphysical (Eugene van der Pijll and Philippe Bruhat (BooK)) - the Pataphysical calendar, an artificial calendar that has 13 months of 29 days each.
• DateTime::Fiction::JRRTolkien::Shire (Tom Braun and Tom Wyant) - the calendar used by those nasty hobbitses.

• DateTime::Event::Cron (Matt Sisk) - generates recurrences from cron lines/files.
• DateTime::Event::Easter (Rick Measham and Jean Forget) - calculates Easter events (Easter Sunday, Palm Thursday, etc.) for both Western churches and the Eastern Orthodox church.
• DateTime::Event::ICal (Flavio Glock) - create recurrences in the same way as iCal.
• DateTime::Event::NameDay (Ben Bennett) - get day names by date or name for various countries.
• DateTime::Event::Random (Flavio Glock) - create sets of random datetimes.
• DateTime::Event::Recurrence (Flavio Glock) - a convenient interface for generating sets to represent various types of recurrences, such as monthly, daily, etc.
• DateTime::Event::Sunrise (Ron Hill and Jean Forget) - calculates sunrise and sunset for places around the world.
• DateTime::Event::Zodiac (Maros Kollar) - calculates (astronomically correct) zodiac signs

• DateTime::Fiscal::Retail454 (Jim Bacon) - Provides methods to implement and manipulate 4-5-4 calendars and events.
• DateTime::Fiscal::Year (Jesse Shy) - provides calculations for things like "quarter of the fiscal year".

• DateTime::Functions (Autrijus Tang and Neil Bowers) - a procedural interface for DateTime constructors.
• DateTime::HiRes (Joshua Hoblitt) - provides a single method, DateTime::HiRes->now(), that uses Time::HiRes::time() to get the current epoch value as a floating point number.
• DateTime::LazyInit (Rich Measham) - an object with a very quick constructor that provides a small part of the DateTime.pm API, inflating into a real DateTime.pm object as needed .
• DateTime::TimeZone::Alias (Joshua Hoblitt) - allows you to create and remove arbitrary aliases to time zones. This functionality may be integrated into DateTime::TimeZone in the future.
• DateTime::TimeZone::LMT (Rick Measham and Jean Forget) - a time zone class for local mean time offsets.
• DateTime::TimeZone::TAI (Eugene van der Pjill) - implements support for TAI as a timezone.

Modules that are not officially part of the DateTime suite but which play nice with it in some way.

• Class::DBI::Plugin::DateTime
• Data::FormValidator::Constraints::DateTime
• Data::iCal::DateTime
• HTML::CalendarMonth
• Rose::DateTime
• Sort::Key::DateTime
• Template::Plugin::DateTime (Daisuke Maki) - a Template Toolkit plugin for DateTime.pm objects.
• Time::Format

There are a few modules that start with "DateTime::" that are not trying to be part of the DateTime suite:

• DateTime::BusinessHours
• DateTime::Cron::Simple
• DateTime::Precise - predates the Perl DateTime project - GPS weeks, seconds and days and subsecond precision
• DateTime::Format::Flexible - largely a subset of DateTime::Format::Natural, and not recommended. Use DateTime::Format::Natural instead (and submit patches to improve its parsing if needed ;)

• The following modules are on CPAN, but they are slow and many times produced incorrect behavior. There is work going on to replace these with a better module DateTime::Astro. If you would like to help, please visit the github project page.

• DateTime::Event::Chinese (Daisuke Maki) - calculates events in the Chinese calendar.

• DateTime::Event::Lunar (Daisuke Maki) - calculates lunar events.

• DateTime::Event::SolarTerm (Daisuke Maki) - calculates solar term events.

• DateTime::Util::Astro (Daisuke Maki) - provides functions for calculating various astronomical events that are needed for some calendars.

• DateTime::Util::Calc (Daisuke Maki) - a utility module for calculations that are needed for some calendars.