Hello Sheik,
You may want to look into root cause of why you have a "null" completion date - otherwise it could happen again.
Log an issue with CA Support if needed.
As far as fixing up what you have, you can't do it through the web interface in the default configuration, because Workflow Tasks lock out the Completion Date. But you can do it - with care - through the database or with database utilities like pdm_extract/pdm_load.
CAUTION: Database tools can destroy your database. Try this on a test system before and back up your data.
How to extract all Workflow Tasks associated to a specific Change Order
First identify the Change Order “id” based on the on-screen Change Order “reference number”:
pdm_extract -f "select id, chg_ref_num, summary from Change_Request where chg_ref_num = '2233'"
Then use the Change Order “id” as the Workflow Task “object_id” to extract the Workflow Tasks:
pdm_extract -f "select * from Workflow_Task where object_id = '400651'"
Identify the Workflow Task that has the incorrect Completion Date.
Update the text file so that it just has this one Workflow Task (and headers) in it, with an updated Completion Date.
Then reload it with pdm_load -v -f My_WF_Task.txt
Alternatively, you can go straight in with database tools to the table.
Please see if you can correct the data, and update this thread with the result.
Thanks, Kyle_R.