The SchedulePosts plugin allows you to schedule in advance some posts, so they appear online on the day and time you want them to (and not on the date and time you wrote them).
Deep inside DotClear's configuration (in the dotclear.ini file to be precise) you can specify a time difference between you and your webserver.
When you activate this option in the SchedulePosts plugin, the same time difference is used to calculate the current time before the plugin puts online a new post.
This offers a simple way to avoid putting online posts which you deliberately put offline. The principle is quite simple: if a given post is older than a given number of days, it won't be put online automatically. This setting allows you to define how many days long this time window should be.
If you want to disable this functionnality (ie. every single post in the past whose date has been modified is to be put online), you can set this value to 0.