You can't dynamically change the token value like you're trying to do in the drilldown config. The simplest way to solve this is to update a different token and use that to pass to your dashboard: Example: <drilldown>
<eval token="dd_earliest">$click.value$-300</eval>
<eval token="dd_latest">$click.value$</eval>
<link target="_blank">search?q=index%3D%22xxxx%22%20sourcetype%3D%22xxxx%22%20%7C%20table%20ProcessTime%2C%20FileName%2C%20StartDtTime%2C%20EndDtTime&earliest=$dd_earliest$&latest=$dd_latest$</link>
</drilldown> If it's a deal-breaker to have to manually URL-encode your drilldown search, you could also get the drilldown search to generate it's earliest and latest times dynamically with a subsearch. Set the drilldown time config to global use the folllowing drilldown search: index="xxxx" sourcetype="xxxx"
[| makeresults
| eval earliest=$click.value$ - 300, latest=$click.value$
| fields - _time
| format]
| table ProcessTime, FileName, StartDtTime, EndDtTime
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