计算日期相隔天数

1.使用Date计算

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date beforeToday=new Date();
Date today=new Date();

//format将Date转换为String，parse将日期格式String转换为Date
beforeToday = sdf.parse(sdf.format(new Date()));
today = sdf.parse(sdf.format(new Date())); 

//getTime() : Returns the number of milliseconds since January 1, 1970.毫秒数
long days = beforeToday.getTime() - today.getTime();

//days(毫秒) /1000 = 秒， /60 = 分， /60 = 小时， /24 = 天
days = days / 1000 / 60 / 60 / 24;

2.使用Calendar

Date beforeToday = getDate(); 

Calendar calBefore = Calendar.getInstance();
Calendar calToday = Calendar.getInstance();
calBefore.setTime(beforeToday);

//計算天數
int days = calToday.get(Calendar.DAY_OF_YEAR) - calBefore.get(Calendar.DAY_OF_YEAR);