ensure we set a string in created_at
ensure we set a string in created_at

this should finally close issue #87

zeptolee/HSTracker,mattwelborn/HSTracker,jonnguy/HSTracker,ifeherva/HSTracker,jonnguy/HSTracker,jonnguy/HSTracker,mattwelborn/HSTracker,bmichotte/HSTracker,bmichotte/HSTracker,mattwelborn/HSTracker,zeptolee/HSTracker,mattwelborn/HSTracker,ifeherva/HSTracker