Why is the timestamp of uploaded/downloaded file different from the timestamp of source file?

There are two possible reasons:

• You have not checked Preserve timestamp transfer option (see Transfer options dialog or transfer tab of Preferences dialog)
• WinSCP and the server interpret timestamp differently.

If the latter is true there are two options that you can set in WinSCP to try to make it interpret timestamp in the same manner as your server.

• Option Daylight saving time is useful when timestamp of some files if shifted by one hour
• Option Server timezone offset (SCP protocol only) is useful when timestamp of all files is shifted by the same time span.
• You may need to combine both options in some cases.

Once you set your preferences, save them to stored session.