The date at which the recompression treatment began. The only element inside <dateofrecompressiontreatment> is <datetime>.
<datetime>
(see also <dive>)
<repetitiongroup id="rep345">
    <dive>
        ...
        <hyperbaricfacilitytreatment>
            <!-- start of recompression treatment -->
            <dateofrecompressiontreatment>
                <!-- Date February 4th, 1999 (Thursday) -->
                <datetime>1999-02-04</datetime>
            </dateofrecompressiontreatment>
        <!-- here more statements -->
        ...
        <hyperbaricfacilitytreatment>
    </dive>
    <!-- here can follow more dives -->
    ...
</repetitiongroup>