GuyM, such issue is mentioned much and clear on any Petrel guiding slice or event help file. Be able to know, you enter the setting of the well, click on Time tab>Overide global settings. there are several ways to deal with time depth information:
1. using well checkshot and or using sonic
2. share checkshot from neighbour (offset) well
3. share a time depth relationship TDR from offset well
once u select and click run, means that dataset will be actived, the soft will create a one-way time1 curve, you must manage this curve carefully, you might be confused in case of too many wells in your project. Hope it helps.
Cheers - Junior