There are many dates included in the ISO Metadata Standards and they have several different types - each with its own characteristics. This page has information about valid formats for those dates. Ron Lake's blog includes a helpful description of time in GML.
Date: gives values for year, month and day. Character encoding of a date is a string which shall follow the format for date specified by ISO 8601. A full date is formatted as YYYYMMDD or YYYY-MM-DD. This type is used in the following fields:
DateTime: combination of a date and a time type (given by an hour, minute and second). Character encoding of a DateTime shall follow ISO 8601. Combined dates and times should be formatted as YYYYMMDDThh:mm:ss, YYYYMMDDThhmmss, or YYYY-MM-DDThh:mm:ss. These representations include no TimeZone indicator, so they are assumed to be local time. YYYYMMDDThh:mm:ssZ would indicate universal time.
<gmd:dateTime> <gco:DateTime>2001-01-01T00:00:00</gco:DateTime> </gmd:dateTime>
Note: This element was incorrectly defined in the ISO 19139 nschema as an xs:dateTime. That type does not allow all of the ISO 8601 options. Specifically, it does not allow the specification of a time range. It will likely be deprecated in the revision of the standard and replaced with stepDateTime.
This type is used only in the CI_Citation and is the only date type that includes a code from the CI_DateTypeCode codelist. Valid values from the CI_DateTypeCode CodeList are: creation (001), publication (002), and revision (003). Note: ISO 19115-1 adds an additional 13 date type code list values to the code list. In figure, see values highlighted in red.
<gmd:CI_Date> <gmd:date> <gco:Date>2000-01-01</gco:Date> </gmd:date> <gmd:dateType> <gmd:CI_DateTypeCode codeList="http://www.isotc211.org/2005/resources/Codelist/gmxCodelists.xml#CI_DateTypeCode" codeListValue="creation">creation</gmd:CI_DateTypeCode> </gmd:dateType> </gmd:CI_Date>