在Android/Java中计算两个日期之间相差天数，可以使用Date类、Calendar类或Joda-Time库。

1.使用Date类：

public static int daysBetween(Date startDate, Date endDate) {
    long start = startDate.getTime();
    long end = endDate.getTime();
    return (int) ((end - start) / (1000 * 60 * 60 * 24));
}

2.使用Calendar类：

public static int daysBetween(Calendar startDate, Calendar endDate){
    Calendar date = (Calendar) startDate.clone();
    int daysBetween = 0;
    while (date.before(endDate)) {
        date.add(Calendar.DAY_OF_MONTH, 1);
        daysBetween++;
    }
    return daysBetween;
}

3.使用Joda-Time库：

public static int daysBetween(DateTime startDate, DateTime endDate) {
    return Days.daysBetween(startDate.withTimeAtStartOfDay(), endDate.withTimeAtStartOfDay()).getDays();
}