These were introduced in 34d01e1b65 to pretty-print certificate validity
ranges. But Microsoft's C runtime took a while to catch up with C99 --
stackoverflow claims that VS2013 and earlier don't support these
specifiers -- so it's possible to end up with PuTTY executables that
misdisplay these dates. Also, the mingw-w64 toolchain's -Wformat
complains about these specifiers, at least on Debian buster, presumably
for the same reason.
Since the specifiers in question have exact pre-C99 replacements, it
seems easiest just to use those.
d2e982efa7