When you have Ruletests in your Corticon.js project, they require adherence to dates and dateTimes that are ISO 8601 compliant, or as the number of milliseconds since the epoch. For testing , the Corticon.js Studio Tester lets you enter a more permissive date and dateTime as Input values; however, the Output values are always strict ISO 8601, as illustrated:

The default date mask in the brms.properties file is: 
Default DateMask to be applied to DateTime values in Tester Output Tree
Default is yyyy-MM-dd'T'HH:mm:ssZ (e.g. 2020-01-01T00:00:00-0000)
com.corticon.javascript.studio.tester.dateformat=yyyy-MM-dd'T'HH:mm:ssZ