DateTime.sol

pragma solidity ^0.8.0;

library DateTime {
    uint256 constant SECONDS_PER_DAY = 24 * 60 * 60;
    int256 constant OFFSET19700101 = 2440588;

    function _daysFromDate(
        uint256 year,
        uint256 month,
        uint256 day
    ) internal pure returns (uint256 _days) {
        require(year >= 1970);
        int256 _year = int256(year);
        int256 _month = int256(month);
        int256 _day = int256(day);

        int256 __days = _day -
            32075 +
            (1461 * (_year + 4800 + (_month - 14) / 12)) /
            4 +
            (367 * (_month - 2 - ((_month - 14) / 12) * 12)) /
            12 -
            (3 * ((_year + 4900 + (_month - 14) / 12) / 100)) /
            4 -
            OFFSET19700101;

        _days = uint256(__days);
    }

    function _daysToDate(uint256 _days)
        internal
        pure
        returns (
            uint256 year,
            uint256 month,
            uint256 day
        )
    {
        int256 __days = int256(_days);

        int256 L = __days + 68569 + OFFSET19700101;
        int256 N = (4 * L) / 146097;
        L = L - (146097 * N + 3) / 4;
        int256 _year = (4000 * (L + 1)) / 1461001;
        L = L - (1461 * _year) / 4 + 31;
        int256 _month = (80 * L) / 2447;
        int256 _day = L - (2447 * _month) / 80;
        L = _month / 11;
        _month = _month + 2 - 12 * L;
        _year = 100 * (N - 49) + _year + L;

        year = uint256(_year);
        month = uint256(_month);
        day = uint256(_day);
    }

    function _isLeapYear(uint256 year) internal pure returns (bool leapYear) {
        leapYear = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
    }

    function _getDaysInMonth(uint256 year, uint256 month)
        internal
        pure
        returns (uint256 daysInMonth)
    {
        if (
            month == 1 ||
            month == 3 ||
            month == 5 ||
            month == 7 ||
            month == 8 ||
            month == 10 ||
            month == 12
        ) {
            daysInMonth = 31;
        } else if (month != 2) {
            daysInMonth = 30;
        } else {
            daysInMonth = _isLeapYear(year) ? 29 : 28;
        }
    }

    function addMonths(uint256 timestamp, uint256 _months)
        internal
        pure
        returns (uint256 newTimestamp)
    {
        (uint256 year, uint256 month, uint256 day) = _daysToDate(
            timestamp / SECONDS_PER_DAY
        );
        month += _months;
        year += (month - 1) / 12;
        month = ((month - 1) % 12) + 1;
        uint256 daysInMonth = _getDaysInMonth(year, month);
        if (day > daysInMonth) {
            day = daysInMonth;
        }
        newTimestamp =
            _daysFromDate(year, month, day) *
            SECONDS_PER_DAY +
            (timestamp % SECONDS_PER_DAY);
        require(newTimestamp >= timestamp);
    }

    function diffMonths(uint256 fromTimestamp, uint256 toTimestamp)
        internal
        pure
        returns (uint256 _months)
    {
        require(fromTimestamp <= toTimestamp);
        (uint256 fromYear, uint256 fromMonth, ) = _daysToDate(
            fromTimestamp / SECONDS_PER_DAY
        );
        (uint256 toYear, uint256 toMonth, ) = _daysToDate(
            toTimestamp / SECONDS_PER_DAY
        );
        _months = toYear * 12 + toMonth - fromYear * 12 - fromMonth;
    }
}