Strange Error in let command

["Com MN PG P E B Consultant 3" <mn-pg-p-e-b-consultant-3.com@xxxxxxxxxxx>]

I have a variable which is guaranteed to either contain a number
(expressing a duration in seconds),
or a number followed by the letter 'm' (expressing a duration in
minutes). My zsh scripts converts
this number so that it always contains the duration in seconds. 

Here is one possibility how to do it:

  delay=20m # For example, duration is 20 minutes
  ...
  delay=$((${delay/%m/*60}))
  # Now delay contains 1200

The trick is to replace a trailing 'm' by '*60' and apply arithmetic
evaluation 
on it. No big deal so far. 

But then I thought that this could be also written by a let statement:

  ((delay=${delay/%m/*60}))

Strangely, this rises the error message

  bad math expression: operator expected at `m'

It is interesting that the very similar statement,

    ((delay1=${delay/%m/*60})) # Stores duration to delay1

DOES work. It is as if in the former case, the occurrence of the word
'delay'
to the left of the equal sign did undergo substitution as well. Is this
a zsh
bug, or did I make a mistake here?

Ronald
-- 
Ronald Fischer (phone +49-89-63676431)
mailto:mn-pg-p-e-b-consultant-3.com@xxxxxxxxxxx