Yup, that time victory is a real pain if you forgot to turn it off. I really don't understand, though, why 2050? In my opinion, that's way too early to be the endgame for a Time Victory. As a noob, I normally start to enter the last era (Information?) around 2020 ish. That means I only have 30 more years to actually do the modern stuff, which is seriously limited. I hope they fix it in Civ VI.
I have been there before. That is just one of the things where you have it happen and then you are sure that it never happens again, until you forget of course.
This is the most annoying part of Civ V for me. I generally always like having a military victory but sometimes I forget to turn off the time victory and screw my entire game up. It feels awful because each Civ V game for me lasts weeks and then I feel like quitting. Thank fully, this has only happened to me thrice. Now I cross check a few times and keep only the military victory option on.
I'd say that nearly every single player that as played this game will have done the exact same thing as you, I know that I have and more than once! While I do obviously see the reason behind the time victory, it can become annoying and I think that it could do with tweaking somewhat. Maybe a double reminder before you start the game just to make double sure that you know you haven't turned it off would help, and while I know that it's not the game developers job to babysit us and make sure that we know what we are doing, with the amount of people that have fallen foul of this (me included remember), I do think that something needs doing.
You live and learn I guess, I'm sure it happened to the majority of the players that play this game a lot. It's something you forget to do in the spur in the moment and it ends up being a big annoyance after. It's safe to say I deleted saves because of this.