**Describe the unexpected behaviour**

Neither of these works:
```
now64(3, 'UTC')
now64('UTC', 3)
```
#include <Functions/extractTimeZoneFromFunctionArguments.h>
#include <Functions/FunctionHelpers.h>
#include <Core/Block.h>
#include <DataTypes/DataTypeDateTime.h>
#include <DataTypes/DataTypeDateTime64.h>
#include <Columns/ColumnString.h>
#include <common/DateLUT.h>
namespace DB
{
namespace ErrorCodes
{
    extern const int ILLEGAL_COLUMN;
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
}
static std::string extractTimeZoneNameFromColumn(const IColumn & column)