Translation:Dad was fine until last week.
"'til" is incorrect. "until" is a compound word formed with obsolete "und" (as far as) + "till". "till" predates "until", not the other way around. "until" has no accepted abbreviation.
Your comment is fascinating given there is no governing body in English, and “correct” has become entirely arbitrary. Each year with increasing dismay I find my students bereft of even rudimentary knowledge of English grammar. You will not find an English teacher in my school who will correct the word “til” in an essay. They’re happy if they discover a semblance of coherence.
The placement of "dernière" decides that. "The last week" would have to be, "la dernière semaine," an event of several weeks duration and he fell ill on the last one. "Last week," being the week prior to this one is, "la semaine dernière." There are several adjectives that change meaning by their placement before or after the word. I learned this de mon ancien prof ancien.