We had someone email us asking for help with configuring a 3550 as an NTP server. The short answer is it can’t be done. The long answer from Cisco’s site is as follows:
The Catalyst 2950, 2955, 3550 and 3560 switches do not have a hardware-supported clock, and they cannot function as an NTP master clock to which peers synchronize themselves when an external NTP source is not available. These switches also have no hardware support for a calendar. As a result, the ntp update-calendar and the ntp master global configuration commands are not available.
(Cisco 1100 APs also do not have a hardware-supported clock - will check on other APs and update)