No, what I meant is that you will not be able to go in future. That is, let's say you played the game until 10th Spring of 4th year, you can go back at any point before that time but never in 11th Spring Year 4, Summer Year 4, etc.
If you complete the game depending on a few factors you can still continue playing trying to complete all side quests. Those timeline with all lovers are not there just those lovers will be available after doing a quest. You will be able to choose which lover you want after that quest. Everything else will remain same.