Donesafe won't let you add an incident that occurs in the future; simply because an incident hasn't occurred until it has. BUT if you're getting this warning and you're NOT trying to add the incident for a future time, then you'll need Donesafe's help with that.
All Donesafe accounts are allocated a general timezone for the whole account. If that timezone hasn't been set correctly, then this could cause incidents caused in your present to be perceived by the system as happening in the future; thus causing the error.
At this stage the only solution for this is to contact Donesafe Support and request that Tier 2 support change the timezone to meet your location. You will need to provide your timezone in GMT for this to be done.