一个月的最后一天？

from datetime import datetime, timedelta

def last_day_of_month(year, month):
    if month == 12:
        next_month = datetime(year + 1, 1, 1)
    else:
        next_month = datetime(year, month + 1, 1)
    return next_month - timedelta(days=1)

# 示例使用
print(last_day_of_month(2023, 4))  # 输出: 2023-04-30

function lastDayOfMonth(year, month) {
    return new Date(year, month + 1, 0).getDate();
}

// 示例使用
console.log(lastDayOfMonth(2023, 4));  // 输出: 30

def is_leap_year(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

def last_day_of_month(year, month):
    if month == 2:
        return 29 if is_leap_year(year) else 28
    elif month in [4, 6, 9, 11]:
        return 30
    else:
        return 31

# 示例使用
print(last_day_of_month(2020, 2))  # 输出: 29
print(last_day_of_month(2023, 2))  # 输出: 28

function lastDayOfMonth(year, month) {
    return new Date(year + (month === 12 ? 1 : year), (month === 12 ? 1 : month + 1), 0).getDate();
}

// 示例使用
console.log(lastDayOfMonth(2023, 12));  // 输出: 31