When setting the expiration date for a cookie, I assume the user’s local timezone is being used.
In the rare occasion of Daylight Saving rollover, it might happen that the clock is set back by an hour it might raise some issues.
Also, my server apps always use UTC time. It might happen that the expiration date is set to a date prior to the user’s system time. And it might happen that the cookie expires before I plan it to be expired.