character string is not in a standard unambiguous format

It might be that you have a character or factor, and it's expecting a numeric vector for conversion from unix time :

as.POSIXct(as.numeric(as.character(df3$deadline)),origin="1970-01-01")

As a suggestion for future debugging, you can check your parameter type by using

class(df3$deadline) 

and making sure you are passing the correct type to as.POSIXlt().

From the help menu for asPOSIX*():

Character input is first converted to class '"POSIXlt"' by 'strptime': numeric input is first converted to '"POSIXct"'. Any conversion that needs to go between the two date-time classes requires a time zone: conversion from '"POSIXlt"' to '"POSIXct"' will validate times in the selected time zone.