#!/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 -r timestamp
timediff=$(( $(date -u +'+%Y%m%d%H%M%S') - $(date -u -d "$timestamp" +'+%Y%m%d%H%M%S') ))
if [ "$timediff" -le 2 ] && [ "$timediff" -ge -2 ]; then
    # don't bother
    exit 0
fi
date -u -s "$timestamp" >/dev/null