library(stringr)
vec <- c("time of the day", "word of the day", "time the day")
str_replace_all(vec, "\\b(?<!\\bof\\s{1,100})the\\b", "of the")
## => [1] "time of the day" "word of the day" "time of the day"
bGlicmFyeShzdHJpbmdyKQp2ZWMgPC0gYygidGltZSBvZiB0aGUgZGF5IiwgIndvcmQgb2YgdGhlIGRheSIsICJ0aW1lIHRoZSBkYXkiKQpzdHJfcmVwbGFjZV9hbGwodmVjLCAiXFxiKD88IVxcYm9mXFxzezEsMTAwfSl0aGVcXGIiLCAib2YgdGhlIikKIyMgPT4gWzFdICJ0aW1lIG9mIHRoZSBkYXkiICJ3b3JkIG9mIHRoZSBkYXkiICJ0aW1lIG9mIHRoZSBkYXkiCg==