bash: let exp ? exp : exp problem

[David selby <debian@pusspaws.net>]

Hello, I am trying to get a bash script to work which will hibernate 
till 1:00 am, it appears to work OK

# ensure script executes next at 1:00am
let _mintoone=(1440-($(date '+%H')*60+$(date '+%M')))+60
if [ $_mintoone -gt 1440 ] ; then
let _mintoone=$_mintoone-1440
fi
sleep $_mintoone'm'

However "info let" seems to say that it has a condensed version of 
if...then...., ie

expr ? expr : expr
conditional expression

So I tried it in various forms ...

let $_mintoone -gt 1440:_mintoone=$_mintoone-1440

but none work. Am I reading the help screen correct ? If so how does the 
syntax work ?
The info on this help screen is a little light.

Many thanks
Dave

PS bash is addictive ...

PPS I am not using cron since the script uses xmessage and executed via 
cron causes xhost access problems, just in case you were wondering ....