#!/bin/sh

# it is in format of `date -u -Iseconds`, example: 2014-09-29T22:59:21+0000
# it comes from dom0, so is trusted
read timestamp
timediff=$(( `date -u +'+%Y%m%d%H%M%S'` - `date -u -d "$timestamp" +'+%Y%m%d%H%M%S'` ))
if [ $timediff -le 2 -a $timediff -ge -2 ]; then
    # don't bother
    exit 0
fi
date -u -s "$timestamp" >/dev/null