Is it possible to call a client script from a "Run server side script" in ATF?

[ANSWER] NO. Also, invoking any client script from a Server Side script is not possible outside the ATF as well

Alternative:

You can simulate the actions necessary in ATF to trigger this client script.

for example :if the client script you are talking about is an Onchange client script that gets triggered when Planned Start Date changes, then you can use the test step "Set Field values" and change the value of Planned Start Date.

Thank you,

Aman Gurram