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There is an issue with Leap Year and DateAdd function that can appear if you want to compare two dates from different years and using some date value from table. For example if you have calendar table and use DateAdd function, you can get 2 rows wit two date for one source date instead of expected 1 row. See bellow reason and solution.

Are you trying convert float to numeric and you get "Arithmetic overflow error converting float to data type numeric" error? This could happen even if all was working before and now it fails. The reason should be in precision and scales settings.