MySQL time difference functions (TIMESTAMPDIFF, DATEDIFF), date conversion calculation functions (date_add, day, date_format, str_to_date)

--0
select datediff(now(), now());

--2
select datediff('2015-04-22 23:59:00', '2015-04-20 00:00:00');

--2
select datediff('2015-04-22 00:00:00', '2015-04-20 23:59:00');

--1
select TIMESTAMPDIFF(DAY, '2015-04-20 23:59:00', '2015-04-22 00:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 00:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 12:00:00');

--2
select TIMESTAMPDIFF(DAY, '2015-04-20 00:00:00', '2015-04-22 23:59:00');

--71
select TIMESTAMPDIFF(HOUR, '2015-04-20 00:00:00', '2015-04-22 23:00:00');

--4260
select TIMESTAMPDIFF(MINUTE, '2015-04-20 00:00:00', '2015-04-22 23:00:00');

-- Use date and string conversion to calculate the first day of the current month and the first day of the next month select curdate() as 'current date', 
DATE_FORMAT(curdate(), '%Y-%m') as 'Current month', 
str_to_date(concat(DATE_FORMAT(curdate(), '%Y-%m'), '-01'), '%Y-%m-%d') as 'The first day of the current month', 
date_add(str_to_date(concat(DATE_FORMAT(curdate(), '%Y-%m'), '-01'), '%Y-%m-%d'), interval 1 month) as 'The first day of the next month';

-- The last day of the current month select last_day(curdate());

-- The first day of the next month select date_add(last_day(curdate()), interval 1 day);

-- What day of the month is the current day? select day(curdate());

-- First day of the month select date_add(curdate(), interval 1-(day(curdate())) day);