Next try, further step:
Now I have a snippet [tt][[firstDate]][/tt] fetching the content of the start_tv building an output as a unix timestamp (e.g. 1216504810 for date of 20.07.08 ...).
In the template of parent travel documents I have a date TV (sortKrit) as unixtime widget with optional and default values:
@EVAL global $modx; return $modx->runSnippet('firstDate');
Putting the TV into the template source as [tt][*sortKrit*][/tt] the rendered page shows the value 1216504810.
Looking into edit mode of a travel document the TV [tt][*sortKrit*][/tt] is "not set".
Therefore Ditto sorting at overview page fails, value of sortKrit is empty. Ditto call:
[[Ditto? &parents=`191` &tpl=`allereisenStart` &display=`all` &dateSource=`sortKrit` &sortBy=`sortKrit` sortDir=`ASC`]]
Why? What’s wrong with this code? Seems that I’m spinning around
) all travel at overview page sorted by the start_tv in grandchild docs. Thanks for ideas.