Question: ISO 8601 representations with reduced accuracy

[ramiromagno]

ISO 8601 allows for "representations with reduced accuracy," meaning that you can omit certain components of the date and time if they are not relevant or available. Here are some examples:

Date Only:
    YYYY-MM-DD (complete)
    YYYY-MM (reduced accuracy, no day)
    YYYY (reduced accuracy, no month or day)

Time Only:
    HH:MM:SS (complete)
    HH:MM (reduced accuracy, no seconds)
    HH (reduced accuracy, no minutes or seconds)

Combined Date and Time:
    YYYY-MM-DDTHH:MM:SS (complete)
    YYYY-MM-DDTHH:MM (reduced accuracy, no seconds)
    YYYY-MM-DDTHH (reduced accuracy, no minutes or seconds)

Is this supported in lubridate? I mean, is there a way to generate ISO 8601 strings with missing components, and indicate how those missing components are represented, e.g. with an hyphen? Are lubridate functions' implementation, in one way or the other, always going through strftime() or strptime()? As far as I understand these functions always impute the missing values to 0. Is there any low-level parser that collects all date-time components into a list before missing elements are converted to default values?

Here are examples of ISO 8601 dates or date-times that illustrate what I would like to achieve:

From	Precision	To
December 15, 2003 13:14	Unknown seconds	2003-12-15T13:14
December 15, 2003 13	Unknown minutes and seconds	2003-12-15T13
December 15, 2003	Unknown time	2003-12-15
December, 2003	Unknown day and time	2003-12
2003	Unknown month, day, and time	2003
Between 10:00 and 10:30 on the morning of December 15, 2003		2003-12-15T10:00/2003-12-15T10:30
Between the first of this year (2003) until "now" (February 15, 2003)		2003-01-01/2003-02-15
December 15, 2003 ??:15	Unknown hour with known minutes	2003-12-15T-:15
December 15, 2003 13:??:17	Unknown minutes with known date, hours, and seconds	2003-12-15T13:-:17
The 15th of some month in 2003, time not collected	Unknown month and time with known year and day	2003---15
December 15, but can't remember the year, time not collected	Unknown year with known month and day	--12-15
7:15 of some unknown date	Unknown date with known hour and minute	-----T07:15