No 'next-week' function

[Akira Kyle]

I've been transitioning to guix and was trying to setup some 
automatic backup scripts at daily, weekly, and monthly 
intervals. However, it seems while the documentation lists 
'next-week' and 'next-week-from' as valid functions to specify the 
job interval, it appears that they are not implemented. It would 
be great to have such functions available. In the meantime 
'(next-day '(1 8 15 22 29))' seems like it'll be sufficient for my 
purposes.

Thanks!