How NMSP handles time zone for data fields?
Notes and SharePoint both store date/times as UTC. In the case of Notes, the date/times are displayed according to the current Windows location settings; in the case of SharePoint, the date/times are displayed according to the locale settings for the SharePoint site.
So, in order for the displays to match, the Windows setting for the machine running the Notes client should match the locale setting for the site to which the data has been migrated.
If you have a document that has time - 5am 4/23/2012 (int GMT +5.5 timezone) and you migrate it to a site that is in GMT -6.0 time zone than it's expected behavior that you see the time (and date) difference. The document in SharePoint will have 6pm 4/22/2012.
If you change time zone for the site to GMT +5.5 you should see the correct time.