How do I merge multiple dates backup from Time Machine?
Don't worry about it.

Time Machine handles all this itself without any user intervention. It keeps several backups per day, then one a week, then one a month… right back to the start of the backup.

It does this without using any extra disk space, but making hard links to any files which appear to be duplicated. This means that there is really only one file, but many places you can see it.