<quote
cite="http://www.w3.org/TR/2006/WD-InkML-20061023/#trace">

duration = xsd:decimal
The duration of this trace, in milliseconds.
Required: no, Default: unknown

timeOffset = xsd:decimal
The relative timestamp or time-of-day for the start of this trace, in
milliseconds.
Required: no, Default: unknown

</quote>

'undefined' is not a legal value of xsd:decimal

You need to roll this out in different prose.  When the attribute
is not present, no information is given as to the time that this
ink appeared.  'Default' has a slightly different semantic than
this,  If there is a default, it is a legal value that is true even
when stated.

Suggestion:

For @duration, let the default be zero.  (data valid at one time)

For @timeOffset, say Default: none.

Al