When should commas be used with introductory phrases that include dates?

A comma may generally be omitted from an introductory phrase of two or three words, but consider using a comma when you wish to emphasize the phrase. The following sentences contrast two dates, so setting each date phrase off with a comma calls the reader’s attention to the dates:

In 2007, few people used smartphones. By 2010, smartphone use had become widespread. 

Read more about deciding when to use commas.