Maybe it's a lawyer thing but I quite frequently need to calculate dates. For instance, I need to figure out what day is 30 days after I serve documents on somebody or how many days elapsed between date X and date Y.
I was thinking this would be really convenient if I could do it with Siri but the Siri documentation doesn't say anything about calculating dates. After a little experimentation, however, I stumbled upon the necessary syntax:
"What is X days from Y date."
For example, "What is 30 days from June 1, 2013."
It also works in reverse.
"What is 30 days before June 1, 2013."
Figuring out days between dates also requires a specific syntax.
"How many days between June 1, 2012 and July 1, 2013?"
As you can see, this search returns a Wolfram Alpha page, which isn't as pretty but includes more details. Once you internalize the syntax, you'll find there is no faster way to calculate dates than with Siri.