Indeed the price template sensor could be used to “dynamically” determine the price to be used with if statements for when date is x.
However, it would be easier (next to possible reverse fixing pricing in case your to late) to have an option in the energy configuration to mark an new static tariff with an start date.
(Would be more user friendly less technical people like my parents. Who are able to perform basic tasks via the UI if explained but who I wouldn’t want anywhere near the yaml configuration to reduce the number of support calls)
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