السَّاعَة here refers to the Hour of Resurrection. The answer about its time is different from the answer about أَمَارَات, its signs.
The two questions are:
1. مَتَى السَّاعَة؟ — When will the Hour occur?
2. مَا أَمَارَاتُهَا؟ — What are its indications?
Knowing some signs does not amount to knowing a calendar date. Nothing in this reply permits a reader to calculate the exact time from a building project or a social change.