How to properly define custom event time-stamp fields in ETW, to be used as StartTime and EndTime for gannt graph in WPA?

Question

I'm interested in getting custom ETW events to display Start Time and End Time, like some default windows events do, in order to present it in gantt form, like so:

I've hacked around Bruce Dawson's demo manifest and multi-provider files, to add Start Time and End Time fields to one of the event templates.

However, I can't get it to even recognize the field as a Time field.

I compose the template for the event payload as described in Microsoft's specification as follows

<data inType="win:FILETIME" outType="xs:dateTime" name="Start Time" />           
<data inType="win:FILETIME" outType="xs:dateTime" name="End Time" />

And convert my QueryPerformanceCounter variables accordingly :

//start is LARGE_INTEGER
//end is LARGE_INTEGER

FILETIME ftStart;
ftStart.dwLowDateTime = start.LowPart;
ftStart.dwHighDateTime = start.HighPart;

FILETIME ftEnd;
ftEnd.dwLowDateTime = end.LowPart;
ftEnd.dwHighDateTime = end.HighPart;

WPA says "Unable to parse data" in the relevant fields:

Then I look at Microsoft WPA documentation:

If the graph is a Gantt chart, each column that you move to the graphing elements area must contain only timestamp values. A mark in one of the horizontal bars in the Gantt chart represents one timestamp value.

So I look and see that timestamps are simply LARGE_INTs so I go back to original format and write the manifest like so:

<data inType="win:UInt64" outType="win:ETWTIME" name="Start Time" />           
<data inType="win:UInt64" outType="win:ETWTIME" name="End Time" />

Then the data is presented as a large count of nanoseconds instead of time. And I'm unable to select either field as "Start Time" or "End Time".

What am I doing wrong?