R. Convert TimeStamp column from DataFrame to Date Format column

Question

I have got a question.

There are a column with timestamp records like '1643410273'(summary more than 1.2M records). How can I transform it into Date format?

I created this code (R Language):

mydata <- read.csv("summary_dataset.csv")
unique(mydata$Callsign)
flight <- mydata[mydata$Callsign == "AFR228",]
AltitudeValue <- flight$Altitude
UTC_Timestamp <- flight$Timestamp
Flight_Date <- vector()

for (i in 1:length(UTC_Timestamp)){
  Flight_Date[i]=as.POSIXct(UTC_Timestamp[i], origin='1970-01-01', tz="UTC")
}

Flight_Date

But, in result, vector Flight_Date was filled Timestamp records. What's wrong?