[Remind-Fans] Forward scanning, how?

[David]

Hello Remind-fans

I have a simple newbie question. I've read the manpage and all the
other references at http://www.roaringpenguin.com/products/remind
Remind is so powerful that I feel I must have overlooked something
obvious. I apologise if the answer is in a FAQ somewhere that I
missed.

What I seek is similar to "backward scanning" but I want to go
*forward* in time.

For example, how would I generate a reminder 30 days following the
17th day of each month.

I tried this:
REM [trigger(evaltrig("17")+30)] MSG - NO OUTPUT in tkremind

so I did a test like this:

REM [trigger(evaltrig("17")-1)] MSG - WORKS in tkremind - reminder on
16th of each month
REM [trigger(evaltrig("17")+1)] MSG - NO OUTPUT in tkremind

So I have two questions:
1) any way to achieve the desired result?
2) why did my test succeed with -1 but fail with +1 ?

Many thanks for any replies to either question.
David