The article Editing Skype history / messages / database has this solution :

1. Download SQLite Database Browser
2. While Skype is closed, open its database at
   C:\Users\<Windows-user-name>\AppData\Roaming\Skype\<Skype-user-name>\main.db
3. Display the messages from December by entering :
   select * from Messages where timestamp >= 1421020800
4. Move these messages to September 1st by entering :
   update Messages set timestamp = 1420761600 where timestamp >= 1421020800
5. Click "Write changes"

The above timestamp numbers were calculated using the website Unix Time Conversion page. Using this page I calculated the dates as follows :

1 December 2015 = 1421020800
1 September 2015 = 1420761600

Based on the article Editing Skype history / messages / database, use this procedure :

1. Download SQLite Database Browser
2. While Skype is closed, open its database at
   C:\Users\<Windows-user-name>\AppData\Roaming\Skype\<Skype-user-name>\main.db
3. Display the messages from December by entering :
   select * from Messages where timestamp >= 1421020800
4. Move these messages to September 1st by entering :
   update Messages set timestamp = 1420761600 where timestamp >= 1421020800
5. Click "Write changes"

The above timestamp numbers were calculated using the website Unix Time Conversion page. Using this website page, I calculated the dates as follows :

1 December 2015 = 1421020800
1 September 2015 = 1420761600

The article Editing Skype history / messages / database has this solution :

1. Download SQLite Database Browser
2. While Skype is closed, open its database at
   C:\Users\<Windows-user-name>\AppData\Roaming\Skype\<Skype-user-name>\main.db
3. Display the messages from December by entering :
   select * from Messages where timestamp >= 1421020800
4. Move these messages to September by entering :
   update Messages set timestamp = timestamp - 259200 where timestamp >= 1421020800

The above timestamp numbers were calculated using the website Unix Time Conversion page. Using this page I calculated the dates as follows :

1 December 2015 = 1421020800
1 September 2015 = 1420761600
(1 December 2015) - (1 September 2015) = 259200

I remark that I do not have the environment for testing the above, so cannot guarantee that this procedure and the product SQLite Database Browser do work as described.

The article Editing Skype history / messages / database has this solution :

1. Download SQLite Database Browser
2. While Skype is closed, open its database at
   C:\Users\<Windows-user-name>\AppData\Roaming\Skype\<Skype-user-name>\main.db
3. Display the messages from December by entering :
   select * from Messages where timestamp >= 1421020800
4. Move these messages to September 1st by entering :
   update Messages set timestamp = 1420761600 where timestamp >= 1421020800
5. Click "Write changes"

The above timestamp numbers were calculated using the website Unix Time Conversion page. Using this page I calculated the dates as follows :

1 December 2015 = 1421020800
1 September 2015 = 1420761600