Whilst I don't have the Ixus 135, I have had, and continue to have quite a bit of Canon photographic machinery. Infuriating as it possibly is, I've always found that if it's mentioned in the manual, you must be able to do it somehow.
I notice that this setting this "feature" comes within the "Smart Auto Mode" section of the manual. You've possible checked on this, but perhaps make sure that it actually is in that mode when you look for this option. If both this and the image resolution options are 'hiding' there must be some secondary option which is preventing this.
I mention this because I've occasionally found my Canon SLRs to deny access to a desired setting. I've discovered that some of these are mode specific. (I've never used the time and date stamp so I've never had to turn it "off").
Persevere - it must be possible - it's just that some other prior setting or mode is apparently concealing access to those particular screens.
And I agree that sometimes the menu system is not particularly intuitive - not for me anyway.
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