ivoa: DM type system

[CresitelloDittmar, Mark]

Gerard,

I was thinking about this more on the way home yesterday..  I don't think
datetime can handle all three cases.
  + it certainly handles DATE-OBS, as an instant in time (date+time)
  + it may handle MJDREF, as it could be considered different
representation of a date+time as 'MJD' date.

  + TSTART (TIMEOFFSET)  requires additional information (the time0 of the
frame) to determine, and can have a variety of units [ms, s, d, m, y, ... ]
which the others don't.
      while the first two represent an instant in time, this is really a
span of time, which can be converted to an instant if you know the start
point.
      Yes.. that is the same as MJDREF, but MJD has a fixed, known start
point while this one is arbitrary and cannot be internalized to the type.

So, I still think we need both.

Mark



On Tue, Apr 18, 2017 at 11:07 AM, Gerard Lemson <gerard.lemson at gmail.com>
wrote:

> Hi Mark
>
> On Mon, Apr 17, 2017 at 11:51 AM, CresitelloDittmar, Mark <
> mdittmar at cfa.harvard.edu> wrote:
>
>>
>>
>> ...
>>>
>> I don't think this needs to be complicated, so let's put these on the
>> back burner.
>>
>> Thanks!
>
>>
>> >>     * Quantity and datetime would not have a common ancestor, so we
>> could not define a TimeStamp
>> >>       which would allow time represented as a RealQuantity OR datetime.
>> >>       https://volute.g-vo.org/svn/trunk/projects/dm/STC-
>> <https://volute.g-vo.org/svn/trunk/projects/dm/STC-2.0/doc/diagrams/alt/temporal%20domain%20diagram.png>
>> 2.0/doc/diagrams/alt/temporal%20domain%20diagram.png
>> <https://volute.g-vo.org/svn/trunk/projects/dm/STC-2.0/doc/diagrams/alt/temporal%20domain%20diagram.png>
>> >>
>>
>> >So are you saying you'd like a DatetimeQuantity? With only a "datetime
>> unit" being sufficient?
>> >I had hoped   'ivoa:datetime' would be sufficient and the precise
>> representation would be left to the mapping.
>>
>> This sounds like you are suggesting that 'ivoa:datetime' should suffice
>> for all uses.
>>    MJDREF  = 50814.02               / [d] zero point for times - MJD
>>    TSTART  = 84244214.7546979934 <(754)%20697-9934>    / [s] Observation
>> start time
>>    DATE-OBS= "2000-09-02T01:10:14"  / Date and time of observation start
>>
>> I want all of these to be a TimeStamp type (abstract).
>>   * datetime = has vodml/html description:  "Represents a moment in time
>> using a date+timestamp."
>> which covers 'DATE-OBS', the other 2 are satisfied by RealQuantity.
>>
>> But.. since RealQuantity and datetime do not have a common ancestor, I
>> cannot define TimeStamp without either an anyType or a DateQuantity.
>>
>>
> Yes, that is correct, but I see now (I think) what might cause confusion,
> namely the description of  ivoa:datetime in the model as
> "Represents a moment in time using a date+timestamp". What was meant there
> is that it is not just a day/date (2017-04-18), and not just a time
> ("13:00:01.3"), but the fully specified moment in time.
> In the current description it does indeed sound as if date and time must
> be represented, i.e. too much like a prescription for valid serializations.
> This was NOT intended.
> ivoa:datetime is supposed to represent the concept of a Timestamp as you
> mention it, and it should be up to serialization prescriptions to take care
> of valid representations.
>
> So if that interpretation is given to ivoa:datetime, would that type be
> sufficient for your requirements?
>
> Cheers
> Gerard
>
>
>
>
>> Mark
>>
>>
>>
>
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