The law requires all employees to provide their employers with advance notice of military service, but does not specify a particular length for the notice period. Circumstances arise, especially in a mobilization scenario, when you may have very little advance notice from military authorities. USERRA's purpose would be defeated if the lateness of the notice to the civilian employer prevented you from later becoming reemployed, especially when you had little or no notice from the military. Notice may be either written or oral; a copy of their military orders is not required by the law. Notice may be provided by the employee or by an appropriate officer of the branch of the military in which the employee will be serving. However, no notice is required if:   military necessity prevents the giving of notice (for the purpose of this exception, the Department of Defense defines military necessity as a mission, operation, exercise or requirement that is classified, or a pending or ongoing mission, operation, exercise or requirement that may be compromised or otherwise adversely affected by public knowledge.); or; the giving of notice is otherwise impossible or unreasonable.   The Department of Defense strongly recommends advance notice to civilian employers be provided at least 30 days prior to departure for uniformed service when it is feasible to do so.