Shortcuts for deferring 30 minutes?

In defer, there are shortcuts that I can use. e.g.

1h -> 1 hour later
1d -> 1 day later
1w -> 1 week later

How do you do minutes? 30m leads to 30 months later

I guess you can only type dates into the defer field. You can vote for a feature request by e-mailing to suggest changing this.