The same thing happened with the A Team 

In the show they were Vietnam vets 

In the 2010 Liam Neeson film they were Iraq vets 

Captain America went into freeze at the end of WW2 so can wake up Now, whatever year Now is 

Punisher has to be updated to have been in the most recent war 

Batman, Superman. Etc They all keep the same fixed time, ages, but it is always the current year 

If Buffy was made today they would all have mobile phones and be on Facebook and Tik Tok