<datetime> generally indicates a date and time after the conventions of the International Organization for Standardization (ISO), ISO 8601 (see also for example http://en.wikipedia.org/wiki/ISO_8601, and http://de.wikipedia.org/wiki/ISO_8601). The indication consists of a four-digit year number, a two-digit month number (with leading zero, if necessary), and a two-digit day number (with leading zero, if necessary). Year, month, and day numbers may be separated by a hyphen "-" for better human readability. The date can be succeeded — separated by the upper letter "T" (for "Time") or simply by a space " " — by a time designation. Hours, minutes, and seconds respectively, are given as a two-digit number (with leading zero, if necessary). Again, for a better human readability, hours, minutes, and seconds may be separated by a colon ":". Finally, the time designation may be followed by the upper letter "Z" (for "Zulu Time") to indicate coordinated universal time (UTC), or — indicating this more precisely — by an "+" or "-" followed by a two-digit hour number, and a two-digit minutes number, if necessary (optionally separated by a colon for better human readability).
Not in every case the complete date and time is known. In these cases it is possible to omit lower order time/date elements. If only time is given, it must be preceded by a "T".
Examples:
February, 1985 -> 1985-02 or 198502
January 5th, 1997 -> 1997-01-05 or 19970105
October 25th, 2008, 16:05 hour UTC -> 2008-10-25T16:05 or 2008-10-25T16:05Z or 2008-10-25T16:05+00:00 or 2008-10-25 16:05 or 20081025T1605 or 20081025T1605+0000
9:47 hour -> T09:47 or T0947 (the "T" must be given in this case — otherwise without "T" in the format T0947 it would indicate the year 947)
<datetime> is compulsory in that cases, if it is the only child element of its parent element: <dateofflight>, <dateoftrip>, <issuedate>, <sunk>, and <validdate>. It also must be given inside <divecomputerdump> (though there is a second element <dcdump> — which also is compulsory). Additionally, <datetime> is compulsory inside <informationbeforedive>, of course. In all other parent elements <datetime> can be given, but is not mandatory.
<dateofflight>, <dateoftrip>, <divecomputerdump>, <examination>, <generator>, <imagedata>, <informationbeforedive>, <issuedate>, <purchase>, <rating>, <sunk>, <validdate>
(see also <examination>, or <imagedata> e.g.)
<repetitiongroup id="rep_23">
<dive>
<informationbeforedive>
<!-- Date April 6th, 1999 (Tuesday), 10:24 hours -->
<datetime>1999-04-06T10:24</datetime>
<!-- here more statements -->
...
</informationbeforedive>
</dive>
<!-- here can follow more dives -->
...
</repetitiongroup>