What 'calculator step’ did you try? Im a bit confused as to what your problem is.
Are you giving the LLM a date (1/29/26 for example) and then asking it to find out what day of the week that is?
If thats the case, I would supply a simple tool that is connected to some function that can do this automatically. For example, the LLM calls a tool called something like (date_to_weekday) that takes the full ISO formatted date as input, and the tool outputs the day of the week that would be calculated by deterministic code.
This would prevent any issues with the LLM needing to calculate it on its own and making mistakes.