Coordinated interpretation of the time fields in SIG RRs requires that reasonably consistent time be available to the hosts implementing the DNS security extensions.
A variety of time synchronization protocols exist including the Network Time Protocol (NTP, RFC1305). If such protocols are used, they MUST be used securely so that time can not be spoofed. Otherwise, for example, a host could get its clock turned back and might then believe old SIG and KEY RRs which were valid but no longer are.