Calendar Month vs Month or Calendar Day vs Day

The use of "calendar month" indicates that there are 12 months in a year, and the number of days vary in each month. This is to avoid confusion with lunar months (27-30 days in one month) or computational months (30 days in one month, for ease of calculation).

A "calendar day" means an actual day on the calendar, to avoid confusion with "business/working days."

For example, an employment contract may specify that employees cannot take more than 15 consecutive calendar days in each calendar month. So, it is *OK* for an employee to be off from March 1 through 15, because there are only 15 calendar days. It is *NOT* OK for the employee to be off from March 1 through 16; although that the employee is off for 12 working days, he/she is off for 16 calendar days.

Sometimes, they mean different things. So you must be very careful to choose the right word.

"Calendar month" is from the first day to the last day of a month shown in calendar.
For example (with respect to today, March 14, 2007)
"to pay in this calendar month" means "to pay on or before March 31, 2007"
"to pay in a month" means "to pay on or before April 13, 2007"

"Calendar year" is similar. You must have heard of "academic year" or "fiscal year".

"Calendar year" - from Jan 1 to December 31
"Hong Kong Government Fiscal year" - from April 1 to March 31 the next calendar year.

But we seldom use "calendar day". I think "a calendar day" ends at 12.00 midnight. But a day may mean 24 hours.

Hope this can help you.

2007-03-15 23:25:46 補充：
Thank you, tintin852. Your explanation of "Calendar Day" is very clear.

No it is actually not necessary to do that...
you can write DATE instead...
but still it is not necessary at least.... you have to.. or it is asked to