Yosef Karo, Shulchan Aruch — Yoreh Deah 197
“ Where there is compulsion, for example that she fears dunking at night because of cold or fear of brigands and the like or when the gates of the city are locked, she may dunk on the eighth day while it is still day, but on the seventh day she may not dunk while it is still day even though there is compulsion. ”
