I have two questions: 1. My friend said that you can change the time..

1:Your friend is correect. TO do so you must go to settings on your ds and change the time.

2:Go to Eterna City. Be sure to have someone with cut. Then GO in the forest and you should see some trees that you could cut down. Then cut them down a nd go up and the mansion is right there.

1. I pretty sure you can. I dunno how though

2. It's right inside the eterna city forest entrainec. You have to cut the trees out of the way. You can only catch lv 10 gastly inside the mansion along with rotom.

1. Not in the game, but in the DS system, you can change the time. Just go to settings.
2. If you plan on catching one in the wild, he's extremely rare. Otherwise, you might as well just trade a Haunter and trade back to get a Gengar. Fly to Eterna City and go west. Enter Eterna Forest, and go up. Cut down the trees and continue north to see the Forest Mansion. Go in it, up the stairs, and in the central door [upstairs]. Now, you'll see five rooms. From RIGHT to LEFT, enter the second room. In this room, there should be a painting on the wall with eyes that disappear once you get too close. Save your game, turn of the DS, and insert any GBA Pokemon game you have in the DS, and turn the DS back on and go to Diamond. Run around in this room. You'll most likely run into Gastly, sometimes Haunter, but VERY rarely Gengar.

1. Yes you can change the time, when you turn on you're ds change the time because the Pokemon D/P clock is using the time on your ds

2. Not a clue =D

Well you can change the time by going to your settings on your ds and it will be the same on Pokemon and to get gengar just put any gba Pokemon game in the bottom and go inside the mansion and walk around inside of a room with a picture with a purple person with red eyes that look at you werever you go and walk around till you find gengar and it's not rare I found 1 easily