library(data.table)

				dat <- data.table(a = sample(1:10,5),
                 				  b = sample(1:10,5),
                  				  c = sample(1:10,5))
				apply(dat,2,function(k) { k * (60*60*24) } )-> tmpdata
				for (i in 1:3) {
							dat[,i := tmpdata[1 : NROW(tmpdata),i] %>% as.POSIXct(origin = "1899-12-30",tz = "UTC") , with = FALSE ]
							}