Who -u gives different output if run from cron than if run from terminal?

If I run 'who -u' interactively or from a script invoked through bash in a tty on my Ubuntu 12LTS box I get an output like this:
testuser pts/0 Dec 9 02:32 . 2163 (host.xx.yy)

running the same through cron I get:
testuser pts/0 2012-12-09 02:32 00:05 2163 (host.xx.yy)

I want to check for idle ttys and thus tried to look at field 6 but my script fails because if run through cron I get the PID instead of the idle time.

I can not find anything about output format in the man page of who.
On the info coreutils 'who invocation' I find that the TZ variable affects the output but TZ is not set in my shell nor in the cron environment.
(BTW. I don't like the info pages because I cannot filter them properly)

Is there an easy way to get a standardized output from who?

The local setting for root does not match yours. When crond starts a job it is logged in as root, and uses the environment settings root had at the time crond was started.

crond then execs a process as whoever the crontab is for, but does not include settings.

Login as root run this command

locale

Compare it to the same command you run right now interactively.

Make the two match by adding corrective environment variable settings in your job that runs your script. locale is set by several env variables.

Jim, thanks for pointing me to the right direction.
Indeed locale is set differently for cron compared to my shell environment.
I altered my script so that it runs

LC_TIME=en_US.UTF-8 who -u

instead of a plain who -u   and now the output is well defined.