Here are some time servers that can be used for the master clocks in servers/IMB's:

Google:

While the actual URL for the Google public NTP servers is time.google.com, most servers can't handle a canonical domain name (ie. time.google.com), the actual IP(s) that this maps to is as follows:

time.google.com     219.239.35.0

time2.google.com   219.239.35.4

time3.google.com   219.239.35.8

time4.google.com   219.239.35.12

US Gov't NIST servers: