R ifelse with datatable

Here is my data:

BuyDate       SellDate     Number
2015-01-01    NA           1
2015-01-01    2015-01-03   1
2015-01-01    2015-01-03   -1
2016-12-09    NA           -1

I want to create a new column Start, so I can have the following result.

BuyDate       SellDate     Number    Start
2015-01-01    NA           1         2015-01-01
2015-01-01    2015-01-03   1         2015-01-01
2015-01-01    2015-01-03   -1        2015-01-03
2016-12-09    NA           -1        2016-12-09

The code is:

data[,Start:=ifelse(Number=="1",BuyDate,ifelse(is.na(SellDate),BuyDate,SellDate))]

However, I get:

BuyDate       SellDate     Number    Start
2015-01-01    NA           1         1420070400
2015-01-01    2015-01-03   1         1420070400
2015-01-01    2015-01-03   -1        1420243200
2016-12-09    NA           -1        1481241600

How can I solve this?

str(data)
Classes ‘data.table’ and 'data.frame':
 $BuyDate : POSIXct, format: "2015-01-01" "2015-01-01" "2015-01-01" "2016-12-09"
 $SellDate: POSIXct, format: NA "2015-01-03" "2015-01-03" NA
 $Number  : chr  "1" "1" "-1" "-1"