IF function with dates

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Hello, I deal with parts on trains. For the extra parts NOT installed, we need to inspect them every 28 days to make sure they're still good. For those parts that are installed, no inspection is required. For the non-installed assets, I know the function for G11 will be =F11+28. What do I type in F to make G show up as "N/A" and still keep the date for whenever the part is installed?