I have an answer:

Ma was born on February 29. She was 18 on 1970, which meant she was born on 1952. It was, in fact, a leap year. But 2002 wasn't. No more explanation needed, since the lep years add up exactly to the age. Plus, the clock is changed because of the fact that days get shorter in winter, which is around November to January? (I'm sorry if I'm wrong, I don't get winter in my timezone)

I have an answer:

Ma was born on February 29. She was 18 on 1970, which meant she was born on 1952. It was, in fact, a leap year. But 2002 wasn't. No more explanation needed, since the lep years add up exactly to the age. Plus, the clock is changed because of the fact that days get shorter in winter, which is around November to January, I think.