Important part is that how datetime format is specified in each country based csv file.
Scenario 1
Org time zone is GMT and dataloader timezone is GMT and all the datetime of csv files are in this format
- 2017-04-10
- 2017-04-10 00:00:00
- 2017-04-10T00:00:00Z
- 2017-04-10T00:00:00.000Z
Then you are safe to upload the data.
Scenario 2
Org time zone is GMT and dataloader timezone is GMT
Let's say, check the country data whose timezone is GMT+1 hour is specified as
2017-04-10T01:00:00Z
or
2017-04-10T01:00:00.000Z
.
Then importing file from dataloader will not be an issue.
Salesforce stores the datetime in GMT format and will be shown to the user based on user's timezone.
Scenario 3
If the csv contains the format without any timezone then following data will take GMT timezone:
2017-04-10
or
2017-04-10 00:00:00
So, it will be advisable to match csv datetime zone with dataloader timezone.