I'm sure people are familiar with the "rule" that the tense is moved backward when using reported speech i.e. "I am..." becomes "He said he was..."
However, there are many cases where it is OK not to change the tense e.g. "I'm going on holiday next week" being reported a short time later/the same day as He said he is going on holiday next week, which in my opinion invalidates the original rule as it is an unhelpful attempt to be prescriptive about English usage.
Can anyone offer an intuitive explanation as to why/when it is OK not to change the tense, or perhaps to explain what differences in meaning are implied when each is used?