deals <- read.csv("deals_dates.csv")

Deal_Id  Deal_Name       Start_Date   End_Date
1        Samsung Mobile  3/5/2018     6/23/2018
2        Apple Watch     12/21/2017   2/9/2018

Deal_Id  Deal_Name       Start_Date   End_Date
1        Samsung Mobile  3/5/2018     3/31/2018
1        Samsung Mobile  4/1/2018     4/30/2018
1        Samsung Mobile  5/1/2018     5/31/2018
1        Samsung Mobile  6/1/2018     6/23/2018
2        Apple Watch     12/21/2017   12/31/2017
2        Apple Watch     1/1/2018     1/31/2018
2        Apple Watch     2/1/2018     2/9/2018

library(tidyverse)
df1 %>%
   mutate_at(vars(ends_with("Date")), mdy) %>%
   group_by(Deal_Id, Deal_Name) %>%
   nest %>%
   mutate(data = map(data, ~
       seq(.x$Start_Date, .x$End_Date, by = "1 month") %>%
       {x1 <- floor_date(., unit = "month")
        x2 <-  (x1[-1]-1)
       tibble(Start_Date = c(.x$Start_Date[1], x1[-1]),
              End_date = c(x2, last(.x$End_Date)))})) %>%
   unnest