rails——基于浏览器时区和start_date将时间转换为utc

I have browser time zone in a cookie variable and date in start_date variable as string . say cookie['browser_zone']="Asia/kolkata" & start_date = "2017/12/31 03:00:00" how can I convert above date to with this time zone to UTC time zone.I have browser time zone in a cookie variable a