Click checkbox to auto populate text box with current time

Hello,
Please help. I am looking for a script that can associate a checkbox and a text field formated to display the current time (hhmm 2400 hours format). When a check box is clicked the text box should display the current time as read only. This is for QA and making sure certain tasks are completed within a certain time frame. Is there a code I can use for this? I really appreciate any help you can provide
Thank you !
