Additionally, both "say" and "tell" mean to utter words, but "tell" also means to use those words to give someone information about something. That's why "tell" is used to tell time.
EX: Can you tell me the time? (i.e., Give me that information)
EX: Tell me your name. (i.e., Give me that information)
EX: I can tell by the clouds that it's going to rain. (i.e., the clouds gave me that information.)
EX: She told me to stay home. (i.e., she gave me information, a command)
EX: I can't tell what she'd doing. (i.e., there's no information for me to give)