You can build it from OS.get_datetime:
var time = OS.get_datetime()
var nameweekday= ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]
var namemonth= ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
var dayofweek = time["weekday"] # from 0 to 6 --> Sunday to Saturday
var day = time["day"] # 1-31
var month= time["month"] # 1-12
var year= time["year"]
var hour= time["hour"] # 0-23
var minute= time["minute"] # 0-59
var second= time["second"] # 0-59
var dateRFC1123 = str(nameweekday[dayofweek]) + ", " + str("%02d" % [day]) + " " + str(namemonth[month-1]) + " " + str(year) + " " + str("%02d" % [hour]) + ":" + str("%02d" % [minute]) + ":" + str("%02d" % [second]) + " GMT"