Standardizing Estimation

In order to standardize the Estimation field, we have a workflow rule as follows:

when Estimation.changed && issue.parent for.isEmpty { 
 var interval = created + Estimation; 
 var difference = interval - created; 
 assert (Estimation == 1 hour || Estimation == 2 hours || Estimation == 4 hours || difference.millis % 86400000 == 0): "Permitted Estimations are: 1h, 2h, 4h, and multiples of 8h"; 
}

 After having discovered problems with large estimations, it appears that millis are being calculated incorrectly for >3d.

Is there a better approach?