since 

Identifies the reference point for determining the time of the current
when element, which is obtained by adding the interval to the time of the reference
point. The value should be the ID of another when element within the same parent
element. If the since attribute is omitted and the absolute attribute is not specified,
then the reference point is understood to be the immediately preceding when
element.

Attribute Information

Used By

Element mei:when

Source 

<xs:attribute name="since" type="mei:mei_data.URI">
  <xs:annotation>
    <xs:documentation>Identifies the reference point for determining the time of the current when element, which is obtained by adding the interval to the time of the reference point. The value should be the ID of another when element within the same parent element. If the since attribute is omitted and the absolute attribute is not specified, then the reference point is understood to be the immediately preceding when element.</xs:documentation>
  </xs:annotation>
</xs:attribute>

Sample