How do I add a custom field to a report? Like a text field?

Hi there. Hope you can help me out. I am trying to add a custom field called "start time" (text1=format(start),"hh:mm") to my report, but I cant see this fields in the drop down list of available fields in the report section? Any suggestions on how to accomplish this? I need both start, formatted as day/month, and start time, formatted as hh:mm. Best and thank you so much in advance.