this is actually easy and sraight forward ... if it where dokumented (I'll see to that).
The attribute you need to alter is dueTime in your workitem (activity). In the cases of the current activity that would be $.activity('this').dueTime.
in the following example i included moment js and used it for easy time handling to set the dueTime to tomorrow, same time as now:
load($.LIB_PATH + 'moment.min.js');
$.activity('this').dueTime = moment().add(1,'days').toISOString();