fix datetime: size TimestampToString buffer for years past 9999

[netliomax25-code]

1. TimestampToString formats a time_t with strftime("%Y-%m-%dT%H:%M:%S+0000") into a fixed 25-byte buffer and always returns the first 24 bytes, guarded only by UASSERT(ret == 24) which is compiled out under NDEBUG.
2. For a timestamp in year 10000 or later the %Y field is 5 or more digits, so the output does not fit: strftime returns 0 and leaves the buffer contents unspecified, yet the function still returns 24 bytes of it. That is an uninitialized-stack read in release builds (and the assertion aborts in debug builds).
3. Sized the buffer for any 64-bit year, value-initialized it, and return the exact number of bytes strftime wrote. Output for years up to 9999 is unchanged; added a regression in datetime_test.cpp for a year-10000 timestamp.