I was struggling a bit recently with importing few thousand rows into custom object in SF with Import Custom Objects feature. Everything was fine except of data for Date/Time column where I was always getting "Invalid format" error even though I tried multiple formats (including the one that was provided in files that I exported from SF). I tried also some of the format mentioned in the Data Loader Guide ( http://na1.salesforce.com/help/doc/en/salesforce_data_loader.pdf ). In the end, I downloaded the Data Loader and the same file was imported in a few seconds without any challenges. Kinda weird, I mean - you would expect the Import Custom Objects to support the same date formats as the Data Loader, but well, good news is that it works at least in one product :-)