Why did data warehouse snapshot start CHANGE ?

Investigating some performance matters for our customers in Australia.  I wanted to point out the DW runs and noticed they shifted suddenly on Feb 1 for the repository in question (and a second repository on same lpar).  ntpdate is run hourly on the LPAR.   The Data warehouse snapshot time is the default 24.

Here are dw history for a few of our RTC 4.0.1 stable:

Succeeded    Work Items    Feb 1, 2013 3:12 AM    Feb 1, 2013 3:21 AM    8 minutes, 49 seconds
Succeeded    APT    Feb 1, 2013 3:12 AM    Feb 1, 2013 3:12 AM    7 seconds
Succeeded    Repository    Feb 1, 2013 2:03 AM    Feb 1, 2013 2:04 AM    30 seconds
Succeeded    Common    Feb 1, 2013 2:03 AM    Feb 1, 2013 2:03 AM    6 seconds
Succeeded    SCM    Jan 31, 2013 3:04 AM    Jan 31, 2013 3:04 AM    2 seconds
Succeeded    Build    Jan 31, 2013 3:04 AM    Jan 31, 2013 3:04 AM    2 seconds
Succeeded    Work Items    Jan 31, 2013 1:24 AM    Jan 31, 2013 1:35 AM    11 minutes, 34 seconds
Succeeded    APT    Jan 31, 2013 1:24 AM    Jan 31, 2013 1:24 AM    7 seconds
Succeeded    Repository    Jan 31, 2013 12:03 AM    Jan 31, 2013 12:04 AM    33 seconds
Succeeded    Common    Jan 31, 2013 12:03 AM    Jan 31, 2013 12:03 AM    2 seconds


Succeeded    APT    Feb 1, 2013 2:06 AM    Feb 1, 2013 2:08 AM    1 minute, 54 seconds
Succeeded    Repository    Feb 1, 2013 2:00 AM    Feb 1, 2013 2:01 AM    43 seconds
Succeeded    Common    Feb 1, 2013 2:00 AM    Feb 1, 2013 2:00 AM    8 seconds
Succeeded    SCM    Jan 31, 2013 2:54 AM    Jan 31, 2013 2:54 AM    6 seconds
Succeeded    Build    Jan 31, 2013 2:54 AM    Jan 31, 2013 2:54 AM    10 seconds
Succeeded    Work Items    Jan 31, 2013 12:09 AM    Jan 31, 2013 12:25 AM    16 minutes, 25 seconds
Succeeded    APT    Jan 31, 2013 12:07 AM    Jan 31, 2013 12:09 AM    1 minute, 50 seconds
Succeeded    Repository    Jan 31, 2013 12:00 AM    Jan 31, 2013 12:01 AM    49 seconds
Succeeded    Common    Jan 31, 2013 12:00 AM    Jan 31, 2013 12:00 AM    8 seconds
Succeeded    SCM    Jan 30, 2013 2:56 AM    Jan 30, 2013 2:56 AM    5 seconds

Succeeded    SCM    Feb 1, 2013 4:39 AM    Feb 1, 2013 4:39 AM    13 seconds
Succeeded    Build    Feb 1, 2013 4:39 AM    Feb 1, 2013 4:39 AM    14 seconds
Succeeded    Work Items    Feb 1, 2013 3:42 AM    Feb 1, 2013 4:26 AM    44 minutes, 47 seconds
Succeeded    Common    Feb 1, 2013 2:06 AM    Feb 1, 2013 2:06 AM    8 seconds
Succeeded    SCM    Jan 31, 2013 3:05 AM    Jan 31, 2013 3:05 AM    6 seconds
Succeeded    Build    Jan 31, 2013 3:05 AM    Jan 31, 2013 3:05 AM    9 seconds
Succeeded    Work Items    Jan 31, 2013 2:06 AM    Jan 31, 2013 2:53 AM    47 minutes, 13 seconds
Succeeded    Common    Jan 31, 2013 12:06 AM    Jan 31, 2013 12:06 AM    10 seconds
Succeeded    SCM    Jan 30, 2013 3:06 AM    Jan 30, 2013 3:06 AM    5 seconds
Succeeded    Build    Jan 30, 2013 3:06 AM    Jan 30, 2013 3:06 AM    12 seconds
Succeeded    Work Items    Jan 30, 2013 2:07 AM    Jan 30, 2013 2:55 AM    47 minutes, 35 seconds
Succeeded    Common    Jan 30, 2013 12:06 AM    Jan 30, 2013 12:06 AM    37 seconds

Succeeded    SCM    Feb 1, 2013 4:27 AM    Feb 1, 2013 4:28 AM    20 seconds
Succeeded    Build    Feb 1, 2013 4:27 AM    Feb 1, 2013 4:27 AM    7 seconds
Succeeded    Work Items    Feb 1, 2013 2:25 AM    Feb 1, 2013 2:51 AM    26 minutes, 42 seconds
Succeeded    Common    Feb 1, 2013 2:01 AM    Feb 1, 2013 2:01 AM    2 seconds
Succeeded    SCM    Jan 31, 2013 2:55 AM    Jan 31, 2013 2:55 AM    17 seconds
Succeeded    Build    Jan 31, 2013 2:54 AM    Jan 31, 2013 2:55 AM    7 seconds
Succeeded    Work Items    Jan 31, 2013 12:25 AM    Jan 31, 2013 12:58 AM    32 minutes, 28 seconds
Succeeded    Common    Jan 31, 2013 12:01 AM    Jan 31, 2013 12:01 AM    6 seconds
Succeeded    SCM    Jan 30, 2013 2:56 AM    Jan 30, 2013 2:56 AM    15 seconds
Succeeded    Build    Jan 30, 2013 2:56 AM    Jan 30, 2013 2:56 AM    6 seconds