Hi Rachel,
I don't think there is a field for sent date/time or subject line in the report header. However, you can use the Setup or SmartList in the report to filter your results based on the sent date/time.
You can vote on the idea to add subject line to all email reports here: https://community.marketo.com/MarketoIdeaDetail?id=08750000000IEjBAAW
If you used the email program, you can compare your results on your subject line test and/or sent date/time there: https://community.marketo.com/MarketoArticle?id=kA050000000LGcHCAW