Was just working around the same thing. Turns out no tool is necessary because the Default Variables described in the documentation (https://docs.vapi.ai/assistants/dynamic-variables#using-dynamic-variables-in-a-phone-call) . If you need custom dynamic variables for another reason then I'm not sure but at least for date and time, use {{now}} in your system prompt and identify that as the current date and it'll know for the duration of the call.