data.frame(transdate=seq.Date(as.Date("2016-04-28"),as.Date("2016-05-06"),by="day"),buytime=seq.Date(as.Date("2016-01-01"),as.Date("2016-03-01"),by="week"))->pie

c(format(head(pie$buytime,3),"%b %d, %Y"),format(tail(pie$buytime,6),"%d-%b-%y"))->pie$buytime

as.Date(pie[pie$transdate<as.Date("2016-05-01"),]$transdate,"%b %d, %Y")->pie[pie$transdate<as.Date("2016-05-01"),]$buytime

as.Date(pie[pie$transdate>as.Date("2016-04-30"),]$transdate,"%d-%b-%y")->pie[pie$transdate>as.Date("2016-04-30"),]$buytime

as.numeric(pie$buytime)->pie$buytime

as.Date(pie$buytime,"1970-01-01")->pie$buytime