iCal has a handy feature where you can set the time zone of an event to "Floating". What this means is that the event is no longer tied to a specific time zone. So, let's say you want to schedule lunch at 12pm every day no matter where in the world you are, you set the time zone to floating, and that way you don't get an alert to have lunch at 4am if you're visiting another country.

This class is used to provide the DateTime::TimeZone API needed by DateTime.pm, but for floating times, as defined by the RFC 2445 spec. A floating time has no time zone, and has an effective offset of zero.

If the "name" parameter is "floating", then a DateTime::TimeZone::Floating object is returned. A floating time zone does have any offset, and is always the same time. This is useful for calendaring applications, which may need to specify that a given event happens at the same local time, regardless of where it occurs. See RFC 2445 for more details.

Some observed time values are not related to a specific moment in incremental time. Instead, they need to be combined with local information to determine a range of acceptable incremental time values. We refer to these sorts of time values as "floating times" because they are not fixed to a specific incremental time value. Floating times are not attached and should never be attached to a particular time zone.

Some examples of floating time events include a user’s birth date, a document's publication date, a list of official holidays, or the expiration date for an offer (if not tied explicitly to a time zone).

Example 4:  

Suppose that your application delivers newspapers to users. Your application wants to show the publication date of each issue so that, for example, The Sunday News is always shown as being published on a Sunday. The publication date is thus a "floating time" value.

When comparing times, floating times with no time zone information should use UTC as the time zone.

FORM #1: DATE WITH LOCAL TIME

The date with local time form is simply a DATE-TIME value that

does not contain the UTC designator nor does it reference a time

zone. For example, the following represents January 18, 1998, at

11 PM:

19980118T230000

DATE-TIME values of this type are said to be "floating" and are

not bound to any time zone in particular. They are used to

represent the same hour, minute, and second value regardless of

which time zone is currently being observed. For example, an

event can be defined that indicates that an individual will be

busy from 11:00 AM to 1:00 PM every day, no matter which time zone

the person is in. In these cases, a local time can be specified.

The recipient of an iCalendar object with a property value

consisting of a local time, without any relative time zone

information, SHOULD interpret the value as being fixed to whatever

time zone the "ATTENDEE" is in at any given moment. This means

that two "Attendees", in different time zones, receiving the same

event definition as a floating time, may be participating in the

event at different actual times. Floating time SHOULD only be

used where that is the reasonable behavior.

In most cases, a fixed time is desired. To properly communicate a

fixed time in a property value, either UTC time or local time with

time zone reference MUST be specified.

The use of local time in a DATE-TIME value without the "TZID"

property parameter is to be interpreted as floating time,

regardless of the existence of "VTIMEZONE" calendar components in

the iCalendar object.

絶対時刻であるUtc, Localと概念時刻であるUnspecifiedを同じクラスで扱おうとしたあたりが闇の起源くさいですね。

2015年1月1日00:00:00 という時刻表記は、地球上のどこで読んだかによって意味が変わってしまいます。このような時刻のことを 概念時刻 と呼ぶことにしましょう。ボジョレー・ヌーボーの解禁時刻、11月第3木曜日00:00:00も概念時刻だからこそ世界各地で解禁時刻が違い、バブル期は日本が世界で最初に解禁されるって盛り上がったものです。

概念時刻は、世界のどこのタイムゾーンで扱うかによって別の瞬間を示します。例えば 2017-08-09 18:25:00 は、どのタイムゾーンで解釈するかによって実際の時刻が変わるため、概念時刻です。概念時刻は、理論上は暦(calendar system)に依存していくつかの表現方法がありますが、普通はグレゴリオ暦(西暦)を使います。