That's a good question. Logic says you should be able to just use this:

Between [Start Date] And [Start Date]+[Number of Days]

Access does its best to try to figure out what kind of data you're entering into query parameters. It gets "1/1" as a date and makes it "1/1/20" but unless you specifically cast [Number of Days] into a Long Int then Access treats it as text and it doesn't work.

If you specifically convert [Start Date] to a Date value, and the other to a Long Int, it works:

Between CDate([Start Date]) And CDate([Start Date])+CLng([Number of Days])

This should do it.

Great question. I might make this into a TechHelp video. I'll add it to the list.

Learn More: CLng, CDate, Parameter Query

I was also able to use the commands list below in the query under the Date/Time field:
=[Start Date] and [End Date]  Promptedfor the two dates
Between Date() And Date()  15  Gave the two weeks out from the present date.
I learned a lot from your classes
Tim Schmidt

Glad you're enjoying. :)