be a matter of when, not if

To have a particular outcome that is all but certain, though the date or time at which it comes to pass is not. A: "Are you sure we need this expensive antivirus software? I don't really think we're at much risk." B: "Look, when it comes to cyber attacks, it's a matter of when, not if." I could never understand my parents' need to constantly prepare for the apocalypse. To them, it was always a matter of when, not if, it would happen. She's a toddler, so tantrums are a matter of when, not if. I hope you're up for this.