Estimate field value while uploading a user story via .csv file

[khushboo]

While i am trying to import user stories into JIRA using .CSV file, I found that 1 min = 3600 units. 
I thought JIRA allows entering time in seconds, but this is not true. 
When i enter the original estimate field in csv with 60 or 360, it creates a user story with 0 minute.
When i enter the original estimate field in csv with 3600, it creates a user story with 1 minute.
What is this unit of 1 minute  converted to 3600? 
This is not the case while downloading. When I export user stories with original estimate, i can see that a minute is converted to seconds. 
Seems there is a disconnect on time unit in JIRA. Can you please help me with the answer to my query?

[Nic Brough -Adaptavist-]

Lengths of time in JIRA are stored using milliseconds, but JIRA doesn't display stuff below a second, and rounds up to the nearest minute.  The export is annoyingly inconsistent as it converts them to seconds instead of dumping out milliseconds.

I'd expect anything under 1001 to come out as 0, then from 1001 to 6,000 to be a minute. 

I'm not sure how the rounding works, but at a guess, I think 6,001 -> 12,000 to be 2 minutes.   12,001 -> 18,000 is 3 minutes, and so-on

Then, of course, you've got 360,000 for an hour.