I work in the US and China and travel quite a bit. Currently my screen says 'The Cloud Service will be down from 5:00AM-10:00AM on Sunday, July 26, 2014 for scheduled Maintenance'. I am currently in China and both the Day and Time are ambiguous....You might be tempted to simply use the users local timezone and translate but based on the times that seems unlikely. Its also ambiguous because I don't know if you are doing that.
My suggestion is to BOTH convert the time to the local time found on the user's OS AND explicity add the time zone to the message.