NLM concatenates the following elements to create the journal source for display in PubMed:
<MedlineTA>. Date;<Volume>(<Issue>):<Pagination>. <ELocationID>.  ... where Date is either the <PubDate> or <ArticleDate DateType="Electronic"> element.

The journal article date of publication has traditionally been the date printed on the hard copy of the issue in which the article is included. Since roughly the late 1990s an article can be published electronically first and then later in print, or perhaps only in electronic form. NLM considers electronic publication to be formal publication.  We permit both a Date of Publication (that is still often issue-based) as well as an Article Date (that is specific to the article that is published electronically) for each paper that is cited in MEDLINE/PubMed.  The publisher decides which date is the main date that is included in the journal source after the journal title abbreviation.  If there is another date, it is typically also displayed in the journal source note area after the pagination (or electronic location), using the appropriate label of either Print or Epub.

NLM had documented the various publication models.

The EIdType attribute associated with <ELocationID> is used to label the ID as either a pii or doi (e.g., pii: 18832). When there are both <ELocationID> EIdType = "pii" and <ELocationID> EIdType = "doi", <ELocationID> EIdType = "pii" is presented first.  <ELocationID> may reside in the citation either in lieu of pagination or, for items with both print and electronic locations, in addition to the Pagination element. NLM does not use <ELocationID> EIdType = "pii" in the Source when <Pagination> is present.

Note there is a space after <MedlineTA> and after the period following <Pagination> if <ELocationID> co-occurs with <Pagination>. The punctuation marks (periods, semicolon, and colon) are not found in the elements themselves as physical data but are added for display purposes by programming in NLM search interface(s).  The pii and doi labels are derived from the EIdType attribute value. The Print and Epub labels are derived from the PubModel attribute information. NLM follows the national standard for the formatting of citations for display (American National Standard for Bibliographic References ANSI Z39.29-2005 (R2010), developed and maintained by the National Information Standards Organization (NISO, Z39)).

Examples the from NLM PubMed summary display are:

Dermatol Surg. 2002 Oct;28(10):959-61.
J Foot Ankle Surg. 2002 Sep-Oct;41(5):320-7.
J Am Acad Dermatol. 2002 Nov;47(5 Suppl):S263-5.
Health Serv J. 2002 Nov 28;112(5833):27-9.
Lancet. 2002 Dec 21-28;360(9350):2049-50.
Nurs Times. 2002 Nov 26-Dec 2;98(48):52-3.
Int J Health Care Qual Assur Inc Leadersh Health Serv. 2002;15(6-7):303-10.
Mol Vis. 2002 Dec 20;8:483-93.
Dev Ophthalmol. 2003;36:50-5.
Health Care Financ Rev. 2000 Winter;22(2):189.
N Engl J Med. 2002 Dec 19;347(25):2082-3; author reply 2082-3.
Health Serv Res. 2002 Oct;37(5):xv-xxvi.
Am Fam Physician. 2002 Nov 15;66(10):2008, 2010, 2013.
Soins. 2002 Nov;(670):47-9.
Surg Endosc. 2000 Jan;14(1):86. Epub 1999 Nov 25.
Sci Aging Knowledge Environ. 2004 Jan 28;2004(4):nf13.
Nucleic Acids Res. 2004 Jan 15;32(1):e14.
Nucleic Acids Res. 2004 Jan 16;32(1):380-5. Print 2004.
Acad Emerg Med. 2005 Feb;12(2):129e1-4. [abstract only on page 129 of print journal with full text article on pages e1-e4 in the online version]
Euro Surveill. 2008 Apr 10;13(15). pii: 18832. [ELocationID only; no pagination element]
Br J Pharmacol. 2012 May;166(2):554-6. doi: 10.1111/j.1476-5381.2011.01818.x.
Euro Surveill. 2010 May 13;15(19):pii/19567. [electronic location information submitted as pagination]
Front Genet. 2013 Jan 25;3:330. doi: 10.3389/fgene.2012.00330. eCollection 2012.