Consider sub-second precision

[edwardloveall]

ActiveSupport's TimeWithZone (and ruby's Time) class support sub-second precision if the host system supports it:

require "rrule"
ruby_time = Time.now # 2025-09-17 12:02:59.535244 -0400
Time.zone = "America/New_York"
time_with_zone = Time.zone.now # Wed, 17 Sep 2025 12:03:18.962661000 EDT -04:00

But rrule does not:

require "rrule"
ruby_time = Time.now # 2025-09-17 12:05:43.144258 -0400
rrule = RRule::Rule.new('FREQ=DAILY;COUNT=5', dtstart: ruby_time)
rrule.all
#[Wed, 17 Sep 2025 16:05:43.000000000 UTC +00:00,
# Thu, 18 Sep 2025 16:05:43.000000000 UTC +00:00,
# Fri, 19 Sep 2025 16:05:43.000000000 UTC +00:00,
# Sat, 20 Sep 2025 16:05:43.000000000 UTC +00:00,
# Sun, 21 Sep 2025 16:05:43.000000000 UTC +00:00]

This can cause some subtle bugs if you're expecting times to match exactly. One example would be generating rrule instances from a set time, adding a duration to that time, and trying to find the matching rrule instance:

require "rrule"
Time.zone = "America/New_York"
time = Time.zone.now # Wed, 17 Sep 2025 12:17:33.014852000 EDT -04:00
rrule = RRule::Rule.new('FREQ=DAILY;COUNT=5', dtstart: time, tzid: "America/New_York")
rrule.all[2]    # Fri, 19 Sep 2025 12:17:33.000000000 EDT -04:00
(time + 2.days) # Fri, 19 Sep 2025 12:17:33.014852000 EDT -04:00
rrule.all[2] == (time + 2.days) # false
rrule.all[2] == (time + 2.days).floor # true

You have to .floor the time which loses precision and is likely not obvious to the developer.

[edwardloveall]

Looking into this a bit more and I realized that sub-second precision is not supported by the iCalendar spec. From section 4.3.12:

The "TIME" data type is used to identify values that contain a time of day. [...] The text format consists of a two-digit 24-hour of the day (i.e., values 0-23), two-digit minute in the hour (i.e., values 0-59), and two-digit seconds in the minute (i.e., values 0-60). [...] Fractions of a second are not supported by this format.

The gem could still support this, perhaps optionally, by adding the original sub-second time to the instances:

some_time = Time.now # 2025-09-23 19:30:14.165706 -0400
some_time.subsec # (82853/500000)
Time.new(2025, 1, 1) + some_time.subsec # 2025-01-01 00:00:00.165706 -0500