Soooooooo after watching the movie, I was wondering... what year is it supposed to take place?
In the best case, Jii-chan and the lady were 15-20 years old while the second world war and that's considering his job was allowed at that age, which is probably wrong. The lady is supposed to be in her 70s and Konosuke is supposed to be 61. The 2nd paiting of Van Gogh's sunflowers was burn on the 6th of august of 1945. That means the movie takes place on somewhere between 1985 and 1990? Sorry, the technology displayed doesn't fit.
Even if we took Konosuke's age as non canon, believing that they were so young when it happened and are in their 70s now, that would only give us enough margin to place the movie in the early 2000s, which could actually work since characters are seen using older phones instead of the smartphones some of them were using in more recent episodes of the manga - and specially the anime.