Date field value uploaded using Data Loader is sometimes one day before the date in the file in Salesforce?

The reason for this is that fields such as Close Date are actually date/time fields. When a date is loaded without specifying the time, the time is defaulted to 00:00 – midnight. When another user is in a time zone which is behind the current user’s time zone, the date will show on the previous day. For example:

20 August 2008 00:00 in Paris is 19 August 2008 23:00 in London

Similar issues can arise when daylight savings time begins or ends.

Two simple solutions to this are:
1) Specify a time as well as a date when loading dates using the Data Loader.
or
2) Switch your PC’s time zone to Hawaiian time before starting up the Data Loader.