Possible to add 1 month to header row value when column properties are text/number?

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I've got a project forecasting document where the total value of expected spend is summing up to a row but above that I want to be able to automatically fill in the next 24 months of dates once the starting month is inputted. Right now when using formulas like the one posted here: [how to add 6 months in a date ? — Smartsheet Community] I get the error "#INVALID DATE TYPE". Is there any way around this?


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