The file is always created instantly, the moment a program opens it. It's only filling it with data that takes time. (That is, the process is "open new file - write data - write… - write… - close file". There isn't a "create file with data" function.)
So the timestamps would likely be:
- Created: 12:00 AM (i.e. when the 'open()' function was used)
- Last modified: 12:10 AM (i.e. when the last 'write()' was issued)
In many cases, this applies even when going through a temporary file – most programs create the file only once, and then just move it to the final location when done (which preserves creation time.)
(And if the program copies the temporary file to the destination, then creation time would be "the moment the copying started" – again, the moment the new file was open()'ed.)
If you're particularly curious, you could use Process Monitor to see exactly what happens.