Revert "top level doc.go file"

This reverts commit 73e70c786932d5725631690a3463e2e7aedfe5b5.
Revert "Revert "top level doc.go file""

This reverts commit 4463a9f48fef9d6902dda4fc56bef5087f4b9c03.
add mock clock from time2