See: http://www.openarchives.org/OAI/openarchivesprotocol.html#Datestamp , http://www.openarchives.org/OAI/openarchivesprotocol.html#Dates and http://www.w3.org/TR/NOTE-datetime
The value of datestamps in both requests and responses must comply with the specifications for UTCdatetime in that document. The DRIVER agreement supports the use of optional granularity which involves the time with seconds YYYY-MM- DDThh:mm:ssZ.
This value complies with the specifications for the UTCdatetime in sections 3.3.1 in the OAI-PMH document. Datestamps are encoded using ISO8601 and are expressed in UTC.

<OAI-PMH ...>
  <...>
    <GetRecord>
      <record>
        <header>
          <datestamp>2001-12-14T12:01:45Z</datestamp>

A repository that supports YYYY-MM-DDThh:mm:ssZ should indicate this in the Identify response.

<OAI-PMH ...>
  <...>
  <Identify>
    <granularity>YYYY-MM-DDThh:mm:ssZ</granularity>
<...>