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: