chore: use time.Since instead of time.Now().Sub

[asamuj]

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[rvagg]

Thanks for this @asamuj, some nice cleanups in here. I have a local branch with all of these time.Since fixes, but I was also working my way through some additional staticcheck complaints before I bothered pushing it, but you beat me to this!
My only request is to wind back the 0 -> 0o changes you've made, otherwise I'm happy with this.