I have df:
ID,"address","used_at","active_seconds","pageviews"
71ecd2aa165114e5ee292131f1167d8c,"auto.drom.ru",2014-05-17 10:58:59,166,2
71ecd2aa165114e5ee292131f1167d8c,"auto.drom.ru",2016-07-17 17:34:07,92,4
70150aba267f671045f147767251d169,"avito.ru/*/avtomobili",2014-06-15 11:52:09,837,40
bc779f542049bcabb9e68518a215814e,"auto.yandex.ru",2014-01-16 22:23:56,8,1
bc779f542049bcabb9e68518a215814e,"avito.ru/*/avtomobili",2014-01-18 14:38:33,313,5
bc779f542049bcabb9e68518a215814e,"avito.ru/*/avtomobili",2016-07-18 18:12:07,20,1
I need to delete all strings where used_at more than 2016-06-30. How can I do that?