Trying to search tasks by assignedToID with plannedCompletionDate or status and for some reasons getting empty array though I know there are some tasks. Also I'm wondering if there is a way to search tasks by period of time like 01/01/2019 - 01/14/2019 ? Thank you!
First, assignedToID has a unique property where only the first person assigned to a task or issue will be considered the assignee for that field. See if your objects have multiple assignees that could trigger this issue.
For date ranges, do something like plannedCompletionDate=<start>&plannedCompletionDate_Range=<end>&plannedCompletionDate_Mod=between.
Run a report with the fields you're using and ensure that you are getting the values you expect. Then start modifying your query one variable at a time until your results match.