Applescript - Automator - tomorrow keyboard shortcut
You are trying to add the day in the wrong spot. Try this:

set thedate to ((current date) + (days * 1)) as string
tell application "System Events"
    keystroke thedate
end tell


The "current date" returns the time in seconds, so you have to add one day worth of seconds (86,400 seconds in a day, but I'm sure you knew that) to get tomorrow. Change the multiplier to get more days in the future or past by using a negative value for the multiplier. 

Note that not all days have 86400 seconds (DST change is +/- 3600, Leap seconds, etc…) so on some days it may be an hour off.