**Describe the unexpected behaviour**

Neither of these works:
```
now64(3, 'UTC')
now64('UTC', 3)
```
#include <DataTypes/DataTypeDateTime64.h>
#include <Core/DecimalFunctions.h>
#include <Functions/IFunction.h>
#include <Functions/FunctionFactory.h>
#include <DataTypes/DataTypeNullable.h>
#include <Common/assert_cast.h>
#include <time.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
    extern const int CANNOT_CLOCK_GETTIME;
    extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
}
namespace
{
Field nowSubsecond(UInt32 scale)