Is the kilobyte used by time and ulimit commands either 1000 (SI) or 1024 (old school) bytes?

From man time: M Maximum resident set size of the process during its lifetime, in Kilobytes. From ulimit -a: max memory size (kbytes, -m) unlimited But a "kilobyte" may mean either 1000 or 1024 bytes . I guess here it is a round 1024, but I want to be sure. Authoritative reference would be appreciated.