The timed trials are one of the seven challenges in week 6.You must complete a total of 5 while there are 6 scattered around the map to complete the challenge.The location of those six are:Lonely Lodge(Easy),Retail Row(Easy),[The river near]Lazy Links(requires a golf cart)(Hard),The waterfall at the oasis near Paradise Palms(Easy),for the next one you will need to go beside a river in between Shifty Shafts and Salty Springs(Build a full staircase going around in a circle,if you do not understand,activate the trial and you will see the path to build)(Easy),and the sixth one is where the soccer-field beside Junk Junction used to be in season 4(Hard).I hope this was helpful.