The issue here is that LAST_DAY column should be a POSIXct object and not an int (as opposed to what the value of LAST_DAY should be, 2015-12-01 or -02).
library(dplyr) library(magrittr) df <- data.frame(FIRST_DAY=rep(seq(as.POSIXct("2015-12-01", tz="UTC"), length.out=2, by="days"),2), event=c("a","a","b","b")) df %>% group_by(event) %>% summarise(FIRST_DAY=min(FIRST_DAY), LAST_DAY=max(FIRST_DAY)) -> df_summary df_summary #> Source: local data frame [2 x 3] #> #> event FIRST_DAY LAST_DAY #> (fctr) (time) (int) #> 1 a 2015-12-01 1448928000 #> 2 b 2015-12-01 1448928000 min(df$FIRST_DAY) #> [1] "2015-12-01 UTC" max(df$FIRST_DAY) #> [1] "2015-12-02 UTC" max(min(df$FIRST_DAY)) #> [1] "2015-12-01 UTC"
sessionInfo() #> R version 3.2.3 (2015-12-10) #> Platform: x86_64-w64-mingw32/x64 (64-bit) #> Running under: Windows >= 8 x64 (build 9200) #> #> locale: #> [1] LC_COLLATE=English_United States.1252 #> [2] LC_CTYPE=English_United States.1252 #> [3] LC_MONETARY=English_United States.1252 #> [4] LC_NUMERIC=C #> [5] LC_TIME=English_United States.1252 #> #> attached base packages: #> [1] stats graphics grDevices utils datasets methods base #> #> other attached packages: #> [1] magrittr_1.5 dplyr_0.4.3.9000 reprex_0.0.0.9001 #> #> loaded via a namespace (and not attached): #> [1] Rcpp_0.12.3 assertthat_0.1 digest_0.6.9 #> [4] R6_2.1.1 DBI_0.3.1 formatR_1.2.1 #> [7] evaluate_0.8 httr_1.0.0 stringi_1.0-1 #> [10] lazyeval_0.1.10.9000 curl_0.9.4 rmarkdown_0.9.2 #> [13] devtools_1.9.1 tools_3.2.3 stringr_1.0.0 #> [16] parallel_3.2.3 rsconnect_0.4.1.4 clipr_0.2.0 #> [19] memoise_0.2.1 htmltools_0.3 knitr_1.12
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