doc: update developer notes wrt unix epoch time
doc: update developer notes wrt unix epoch time

Former-commit-id: 6414bec229ebfca40d03ba4df2b4a46e253ed93f
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin