Signature Description
enum class time_frequency : unsigned char {
 annual = 1,
 monthly = 2,
 weekly = 3,
 daily = 4,
 hourly = 5,
 minutely = 6,
 secondly = 7,
 millisecondly = 8,
 // microsecondly = 9,
 // nanosecondly = 10
};
This enum specifies time frequency for index generation and otherwise. The names are self-explanatory.