Goals: Use shared 'months' functions for time (#1082)

Based on Tom's comment
(https://github.com/actualbudget/actual/pull/1058#issuecomment-1568030881

)
I've eliminated all direct use of date-fns and only use the functions in
months.ts.

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Co-authored-by: Matiss Janis Aboltins <matiss@mja.lv>