I had exactly the same problem, that Fedex sent me sms, that they will deliver in 28th, but this morning 24th they sent message that they would deliver today, so I had to cancel my few plans I had and stay at home - also I knew that it would be only my phone, so decided to stay. They delivered to me within 2 hours(because their depot is 20min distance with car from my home), though, even if their webpage said, that they could deliver up to 18:00. So, there were no specific time slots in any of those messages or in webpage, unless we are talking time span 9:00 -18:00, which IMO is not specific at all!
Both of texts had different numbers, so I presume, that it might be possible to call them back and reschedule different time or at least ask about other options. Hey, congrats on getting your phone! 🙂