This is a known issue with the OpenWeather Maps implementation of the Zimmerman method. That API charges a ridiculous amount for historical data and it was wired up to use current weather forecast. We are working to improve this but currently this is not ready yet.
The source for the weather adjustments is available here: https://github.com/OpenSprinkler/OpenSprinkler-Weather
We welcome any pull requests you may have and also allow you to run the service yourself and point OpenSprinkler to it by using http://OSIP/su.