General :: Escape Commands In A Bashrc Alias?

I need to occasionally touch a file with the current timestamp as the filename. I would do so this way:

touch `date "+%Y-%m-%d_%H-%M"`.txt

However, I'd like to define an alias for this. When I try adding to the bashrc this:

alias td="touch `date "+%Y-%m-%d_%H-%M"`.txt"

the result is that the filename is the same during the entire session, since the `date ..` gets calculated just once during login...

How can I get the command to expand only when I call the alias? Or must I use a function for this?