Why has Siri stopped calculating time in minutes upon request?

I ask Siri to calculate time in minutes for me so I can record the amount of time in my work's data entry system. At some point late last week Siri stopped understanding this command. Several times a day I would ask Siri " Calculate the difference in minutes from 8:35 AM to 2:20 PM." It appears that the calculation was aided by Wolfram Alpha and Siri would give me an answer. Now all Siri wants to do with this command (and variations of it) is launch a verbatim Bing search. Can Siri still perform this task? Do I need to phrase my request differently now?