Not sure exactly what you’re seeing, but the lone example on Docs.Splunk for relative_time uses a negative value.
That said, if you want to "look into the future" with your searches, why not just add 1 to argh1 – since you’ve already eval‘d it, you "know" it’s now().
Something like this, perhaps:
| eval argh1=now()
| eval argh2=argh1-1, argh3=argh1+1
from User warren – Stack Overflow https://stackoverflow.com/questions/65359627/splunk-relative-times-with-are-incorrect-in-dashboard-out-by-a-week/65395510#65395510
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