In order to see a 95th percentile value in a exact time frame, how can I set the trending time range to specific date times? e.g. from 11/12/2020 11:00:00 to 18/12/2020 11:00:00
As Ruben already mentioned, you cannot achieve this in the trend component just yet.
However, it's already possible to achieve this using an embedded trend component in the Visual Overview. Using the Range option in the ParametersOptions shape data, you are able to configure an exact time window (see help). Note that you can also use session variables to make it more dynamic.
Configuring the exact start and end time on a Cube trend graph is currently not possible. A new feature to do so has already been requested and accepted, and is currently awaiting integration.
It should be possible though to achieve this through the trend component within Dashboards, in combination with a time range feed.