Before you start, make sure your iPad/iPhone is set to today's time and date.
1) Open Sims, set all your Sims to plant, leave the kids not doing anything for now.
2) Exit the Sims app by double pressing the Home button, holding down the Sims app from the Multi Task bar and tapping on the - red sign.
3) Open Settings. Turn Auto Date/Time off. Change the Month to April
4) Turn Airplane Mode ON. Wi-Fi will turn off automatically.
5) Open Sims app and wait for it to load. You will get an error message and that's okay. Don't tap on RETRY.
6) Go to Settings. Turn Wi-Fi ON. LEAVE AIRPLANE MODE ON. This is very important. If you don't do this the game will crash. Once your Wi-Fi is connected, go back to Sims app.
7) Collect all the stuff you planted. DON'T COLLECT MAILBOX YET. If you do have 10 sims together at one house or empty lot, you will automatically get 10 free LP.
8) Go to Town, collect all your Town's money and XP.
9) Tap on the Party Boat, wait for it to connect. DO NOT COLLECT GIFTS YET. Leave it open.
10) Go to Settings, turn Date/Time back to ON (automatic) and Turn Airplane Mode OFF. This will set your date/time back to today's date.
11) Go to Sims app, collect your gifts from Party Boat and money from Mailbox. You will see your party boat now says 23h 59m instead of being pushed forward thousand of hours.
12) Repeat as necessary.
When you do this it is highly recommended you make sure you're not building anything or it will add more hours to your buildings. If you send your Sims to work, then their time will be off as well... I can't send mine to work until May 29th, I learned that the hard way. This time cheat works for planting, the party boat, and the mail box.
If at some point you messed up and need to reset your party boat again, follow this other post: http://forums.supercheats.com/video-games/the-sims-freeplay/132720/How-to-Fix-Party-Boat-Time-WO.htm