Простой пример (если я правильно понял ваш вопрос):
library(dplyr)
set.seed(123)
df <- data.frame(Name = sample(c("Person1", "Person2"), 6, T),
Date = c("01-31-2019", "02-15-2019", "06-18-2019", "01-31-2019", "02-15-2019", "06-18-2019"),
Sales = runif(6, 10, 100), stringsAsFactors = F)
df$Date <- lubridate::mdy(df$Date)
target <- lubridate::mdy("02-15-2019")
sales <- df %>% filter(between(Date, target - 30, target)) %>%
group_by(Name) %>% summarise(Sales = sum(Sales))