Introduced in #11035

$ cat test.c
#include <stdio.h>
void main() { printf(__DATE__ " " __TIME__ "\n"); }

$ SOURCE_DATE_EPOCH=1 gcc test.c -o test-c
$ ./test-c
Jan  1 1970 00:00:01

$ cat test.d
import core.stdc.stdio;
void main() { printf(__DATE__ ~ " " ~ __TIME__ ~ "\n"); }

$ SOURCE_DATE_EPOCH=1 dmd test.d -oftest-d
$ ./test-d
Dec 31 1969 16:00:01

This is because implementation uses ctime.

The call ctime(t) is equivalent to asctime(localtime(t)).

Because explicitly setting SOURCE_DATE_EPOCH is asking for a fixed time, the implementation should instead be calling asctime(gmtime(t)).