I would start here:
GlideDateTime - ServiceNow Wiki
Once that's familiar, I'd do something like this, pseudocode below
Onchange script:
g_form.hideFieldMsg('impact');
if(newValue < nowDateTime){
g_form.showFieldMsg('datefield','cant select past dates','error');
}
Please note that the above glide date time objects aren't available clientside, so "nowDateTime" will have to come from somewhere.
It's not a bad idea to have a business rule that puts frequently used items on your scratchpad for easy access from the client side.
that looks like this:
onDisplay Business Rule:
g_scratchpad.currDate = nowDateTime();
Then, you retrieve it in the client script:
var nowDateTime = g_scratchpad.currDate;
We have a number of tables with "Scratchpad Management" business rules that expose information we use frequently to client side scripts. In addition to making our lives easier, it allows us to offload much of the querying that is often done in client scripts to the server side.
