
A little bit of background info: Our organization mistakenly used the enterprise trial version of salesforce, then upgraded to the multi-user version. I successfully imported our leads and contacts with dataloader.io. I'm running into problems importing the tasks (past and future) associated with those leads.
I am running into an error on every entry. I'm getting one of 2 errors
1. SOURCE: 'ReminderDateTime' DESTINATION: 'ReminderDateTime' is not a valid DATETIME.[Valid date format: DD/MM/YYYYThh:mm:ss.mmmTimeZone (example: 20/02/1987T12:04:12.000GMT+01:00)].
2. ERROR: invalid cross reference id
These appear regardless of whether I do an insert, upsert or update.
My Data Loader profile is set to YYYY/MM/DD time format in the settings. All dates are entered in YYYY/MM/DDThh:mm:ss format ex. 2014-08-22T15:00:00
The salesforce help and question archives both indicate that this is an accepted format.
3 answers

I'm trying to add the tasks to our existing leads. I thought that since I imported the leads through dataloader, their previous IDs would be preserved, making matching the lead and task IDs automatic. Is that not correct? otherwise I'd have to match the new IDs to the old ones, which would be incredibly time consuming.