newbie q: dicom and datetimes

hi all, i have a newbie question on how dicom is defining datetimes handling/storage.
let's assume this multitime zones scenario: a modality MA sends dicom message from timezoneA to a processor P in timezoneB, and then a qrscp component Q in timezoneB has to respond to a C-FIND request with a dicom message containing the same exam (studies + images), sent by modality MA in timezoneA, to another modality MC which is in timezoneC. what is the time that the exam received by MC should read? in what timezone format it should be? the original timezoneA, the processor timezoneB, the UTC time format? the timezoneC?

the way i see it, the qrscp component is a service, serving DICOM queries, thus returning datetimes in UTC format will alow the requestor to easily translate it in local time. is dicom spefying how datetime supposed to be passed around?

any help, will be appreciated,
thank you!
O.

The Dicom file itself can specify the timezone it was created in.  Make sure that your files include the tag "0008:0201 - Timezone Offset From UTC" and then your application should be able to translate the time if necessary.

swell, now is this tag optional or i can safely assume this tag will be part of the dicom hearders always?

Sorry for not mentioning it earlier.  This is an optional tag, so you cannot make that assumption.  If this time zone translation is important for your application, then you'll need to either write your applications to always have that tag or if you're using some other third party software at some point to add this as a part of your process.

i can not control how the dicom received is created, so it can come with or without that tag. in the case it comes without that tag how should i proceed? should i return the time i have to anyone no matter their time zone? i guess i have no other choice, do i?

Unfortunately if you can't control that the dicom files being created have this tag, then there's not much you can do.  Since it's an optional tag, there's nothing I'm aware of in the Dicom specifications that say how to proceed, so the process is up to you. 

The two options I think of would be to add it so that going forward you can keep track of the time with regard to zones, or do what you suggested and just add/modify times regardless of the time zone.

thanks GregR!