Hi Lynn,
 
Context is everything, so I'm going to assume you are talking about date/time representations in the observation and monitoring schema(s) that you are developing.  The thinking in the collections community has gone something along the lines of this:
 
Concept:  the date-time of a collecting event [= recording-event, gathering-event] -- i.e., when one or more organisms were collected or observed -- expressed in the common Gregorian calendar, in local time.
 
Requirements: 
 
1 - express the data/time exactly as it was recorded
2 - retreive records by date/time ranges (date-time greater than X and/or less than Y)
3 - accommodate date-times of varying precision, including explicit date-time ranges (specifying a duration)
4 - support seasonal (time of year) queries
 
These requirements can be met by the following fields:
 
VerbatimDateTime
EarliestDateCollected
LatestDateCollected
CollectingDatesInterpreted
DayOfYear
 
As John Wieczorek noted, the ISO 8601 format accommodates varying degrees of precision.  Using a "verbatim date-time" [text string] is as good as you can do to satisfy the first requirement, short of scanning field notes or voice recordings.  The earliest and latest dates support the recording of explicit ranges as well as interpreted ranges, which could be used to make "Summer of 1952" retrievable.  The CollectingDatesInterpreted field would be a boolean field, set as true when the earliest and latest dates represent interpretations rather than an explicit range.  The "DayOfYear" field is a compromise that we've offered as a simple way to support queries involving seasonal (annual) cycles; e.g., collected in the northern summer, regardless of year.  But it can be argued that day of year is derivable from the other fields (unless year is unknown), and that it doesn't accommodate explicit ranges. 
 
A bit more documentation is needed to address the odd cases (What do I do when ...?), but these five fields will support a lot of the data exchange needed for this concept.  These fields are not intended to handle the dating of localities in paleontology, nor are they intended to handle named periods that are used in cultural collections (e.g., Iron Age, Victorian). 
 
ABCD uses a few more fields to handle the concept, http://ww3.bgbm.org/abcddocs/AbcdConcepts  but some of these (date, time, and time zone) are handled by the ISO format, except when the larger units are unkown; .e.g., the year is unknown, but the day is [ June 16 ]; or the date is unknown, but the time is [ 14:00-15:30 ].
 

  • AbcdConcept0860  /DataSets/DataSet/Units/Unit/Gathering/DateTime 

  • AbcdConcept0861  /DataSets/DataSet/Units/Unit/Gathering/DateTime/DateText 

  • AbcdConcept0862  /DataSets/DataSet/Units/Unit/Gathering/DateTime/TimeZone 

  • AbcdConcept0863  /DataSets/DataSet/Units/Unit/Gathering/DateTime/ISODateTimeBegin 

  • AbcdConcept0864  /DataSets/DataSet/Units/Unit/Gathering/DateTime/DayNumberBegin 

  • AbcdConcept0865  /DataSets/DataSet/Units/Unit/Gathering/DateTime/TimeOfDayBegin 

  • AbcdConcept0866  /DataSets/DataSet/Units/Unit/Gathering/DateTime/ISODateTimeEnd 

  • AbcdConcept0867  /DataSets/DataSet/Units/Unit/Gathering/DateTime/DayNumberEnd 

  • AbcdConcept0868  /DataSets/DataSet/Units/Unit/Gathering/DateTime/TimeOfDayEnd 

  • AbcdConcept0869  /DataSets/DataSet/Units/Unit/Gathering/DateTime/PeriodExplicit 

    • The ABCD fields pretty much cover the entire concept space.  One could argue whether time-zone is relevant to the description of biological phenomena, but we do know that ship trawls do cross time-zones (including the date-line), and that daylight savings time could stretch or compress some nocturnal collecting events if their durations were calculated too simply.
     
    To some extent these arguments are still going on, so analyze your requirements and your data, then state your position.  ;-)
     
    Cheers,
     
    -Stan

    Stanley D. Blum, Ph.D.
    Research Information Manager
    California Academy of Sciences
    875 Howard St.
    San Francisco,  CA
    +1 (415) 321-8183

    -----Original Message-----
    From: Taxonomic Databases Working Group List [mailto:TDWG@LISTSERV.NHM.KU.EDU] On Behalf Of Lynn Kutner
    Sent: Thursday, February 23, 2006 9:12 AM
    To: TDWG@LISTSERV.NHM.KU.EDU
    Subject: Standards for date / time values?

    Hi -

    I'm working with a suite of date attributes that can include a combination of precise dates, imprecise dates, and ranges of dates (and the same types of time values). We'd like to follow existing standards. If this sort of date / time standard exists, I'd appreciate leads to the appropriate resources.

    Thank you for your help -
    Lynn


    Lynn Kutner
    Data Management Coordinator
    NatureServe
    Email:    lynn_kutner@natureserve.org
    Phone:   (303) 541-0360
    www.natureserve.org