You'd do that as a "validator" probably. There's a plugin module for workflow-functions, including validators.
The basic structure of one of those is a block of logic which would take the entered dates as inputs and return a simple true (pass) or false (validation failed) output, ideally with some text to explain what has failed.
There are a handful of plugins that already provide this sort of function though - see https://plugins.atlassian.com/plugin/details/5048 for one of them
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