[rajath201]

The time_point as/convert adaptors in cpp11/chrono.hpp add std::chrono::seconds(sec) from the wire straight onto the target time_point, so scaling seconds to the Duration's ticks is signed overflow whenever the decoded timestamp doesn't fit. A timestamp96 with tv_sec = INT64_MAX aborts under UBSan (9223372036854775807 * 1000000 in <chrono> on libc++), the largest timestamp64 already overflows a nanosecond clock, and on a picosecond time_point any present-day timestamp32 does.

Route the decode-side additions through detail::checked_add, which range-checks both the scaling and the sum and throws std::overflow_error / std::underflow_error, the same exceptions integral_cast already raises in this adaptor. Floating-point reps keep the old path. The two new tests fail on the current tree and pass with the fix, and the full suite is green with -Werror and UBSan.