The football season officially ended on Sunday. FIFA & UEFA have set calendars for transfer windows, start and end points for the season, international breaks etc. and they updated the international calendar because of coronavirus.
It very much depends on the wording. If it says end of season, Messi's free, if it says July 10th, he's got very little chance.
I expect the wording is related to number of days from the club's last game of the season, or the end of the UEFA calendar. The date August 31st is being bandied about as the deadline according to Messi's camp, it's quite specific.
