**Use case**

Workload generation with continuous queries.

**Describe the solution you'd like**

Similar to now() but non-constant.

namespace DB
{
class FunctionFactory;
void registerFunctionToYear(FunctionFactory &);
void registerFunctionToQuarter(FunctionFactory &);
void registerFunctionToMonth(FunctionFactory &);
void registerFunctionToDayOfMonth(FunctionFactory &);
void registerFunctionToDayOfWeek(FunctionFactory &);
void registerFunctionToDayOfYear(FunctionFactory &);
void registerFunctionToHour(FunctionFactory &);
void registerFunctionToMinute(FunctionFactory &);
void registerFunctionToStartOfNanosecond(FunctionFactory &);
void registerFunctionToStartOfMicrosecond(FunctionFactory &);
void registerFunctionToStartOfMillisecond(FunctionFactory &);
void registerFunctionToStartOfSecond(FunctionFactory &);
void registerFunctionToSecond(FunctionFactory &);
void registerFunctionToStartOfDay(FunctionFactory &);
void registerFunctionToMonday(FunctionFactory &);
void registerFunctionToISOWeek(FunctionFactory &);
void registerFunctionToISOYear(FunctionFactory &);
void registerFunctionToCustomWeek(FunctionFactory &);
void registerFunctionToModifiedJulianDay(FunctionFactory &);