In sourcetree how do i preserve last modified file date across checkin/checkout?

[Gordon Raup]

I go back and forth between branches using Sourcetree.  I'll check in one branch, checkout a second, make some changes, check that branch back in and checkout the first branch again.  The file date on the files that have to be replaced when going back to the first branch are today's date instead of the date the file was last modified.  Is there any way to preserve the last modified date so that it truly is the last time the file was edited?  Thank you.

[lorenzomontesi]

I don't know if git saves last modified date (probably not) but surely sourcetree could set last commit date as last modified date for files, so this feature would be appreciated if it were implemented.