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Your LimeSurvey version: 5.3.3.1[see right hand bottom of your LimeSurvey admin screen]
Own server or LimeSurvey hosting:root server
Survey theme/template:selfmade from fruity
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New survey was created via RPC2 "import_survey" from .lss-file

This PHP-code:

$new_properties = array('startdate'=>$start_dat,'expires'=>$end_dat,'admin'=>$user,'adminemail'=>$email);
print_r ($new_properties);

$survey_status = $myJSONRPCClient->set_survey_properties( $sessionKey, $survey_id, $new_properties);
echo "<br>";
print_r ($survey_status);          

$survey_status = $myJSONRPCClient->set_survey_properties( $sessionKey, $survey_id, array ('startdate'=>$start_dat));   // <<<<<< second try, only startdate
echo "<br>";
print_r ($survey_status);          

produces this output:

Array ( [startdate] => 2022-10-01 00:00:00 [expires] => 2022-11-01 23:59:59 [admin] => xyz [adminemail] => xyz@xxxxx.de )
Array ( [startdate] => [expires] => 1 [admin] => 1 [adminemail] => 1 )
Array ( [startdate] => 1 )

Line 2 of the ouput says startdate didn't work. Looking at the survey from the admin-interface, startdate ist set to 01.07.2016.
After setting the startdate alone again, one can see, that startdate now was accepted.

The second try with "set_survey_properties" and only startdate as parameter was the result of "shoot and hope". Actually, this works for me, but it gives me a bad feeling, not to know why. The same code - whithout the second try - worked well for about 5 years.

Thanks for any comment,
Erich