You may find the situation where AssetAccountant will not allow you to select a prior period to make an adjustment to an asset.
This is most likely because you have taken some actions on that asset that mean you can't adjust prior to that date.
You will need to check all prior events/actions taken on the asset where you are being locked out of making changes.
Typically, this occurs when you have previously made a Revaluation and/or made an Additional Cost to an asset blocking you from applying a prior Adjustment to this.
To address this situation, it is recommended to reverse these prior events (possibly the Revaluation or Additional Cost events), apply the past Adjustment you require and then reapplying the Additional Cost and Revaluation after this date.