This is a common problem, where electronic devices are being retrofitted in place of traditional switches. There is no ready solution to installing the timer you have, other than to upgrade the wiring to bring the neutral and ground to the timer location. Leviton and others do offer some products that don't require a neutral, although they are more fussy about what kinds of loads they can control because the power required to operate the timer has to be obtained via the controlled load and switched hot output. Whether any of these are designed to be safe without a ground is a separate issue.
A US-based poster might have a clearer idea of what products are available where you are (I'm in the UK, we have the same problem but the product