The time range parameter is in seconds, so simply changing that from 21600 to 1800 will do what you need to do. Really simple and configurable for a lot of things in PSKR besides time.
This is what the "Custom map" link in the screenshot you attached, is for.
Simply change the parameter there to create your own custom PSK map with lots of customization possibilities it offers including the "timerange" parameter.
The time range parameter is in seconds, so simply changing that from 21600 to 1800 will do what you need to do. Really simple and configurable for a lot of things in PSKR besides time.
Feature already available.
This is what the "Custom map" link in the screenshot you attached, is for.
Simply change the parameter there to create your own custom PSK map with lots of customization possibilities it offers including the "timerange" parameter.
Closing this request.