Решение, использующее анти-соединение:
library(tidyverse)
#Data
df <- structure(list(Date = structure(c(18302, 18303, 18304, 18305,
18306, 18302, 18304, 18302, 18303, 18306), class = "Date"), Agent = c("A",
"A", "A", "A", "A", "B", "B", "C", "C", "C")), class = c("tbl_df",
"tbl", "data.frame"), row.names = c(NA, -10L)) %>%
mutate(Date = as.Date(Date))
dates <- tibble(Date = seq(as.Date("2020-02-10"), as.Date("2020-02-14"), "days")) %>%
expand_grid(Agent = c("A","B","C"))
dates %>%
anti_join(df) %>%
arrange(Agent)
# A tibble: 5 x 2
# Date Agent
# <date> <chr>
#1 2020-02-11 B
#2 2020-02-13 B
#3 2020-02-14 B
#4 2020-02-12 C
#5 2020-02-13 C