defining a calendar day for stays inUSA

What constitutes a calendar day as defined by IRS and/or US Customs and Immigration? For example , do day trips to shop or play golf to the US count when determining the substantial presence test (31 to 183 days) for Canadians staying the USA in any calendar year?

For pleasure trips to US, any part of the day in US constitutes one day. IRS PUb 519 explains that only days commuting back and forth to work are exmpt, not shopping/golf/vacation.