Validity period is an optional value that can be set on a per message basis to control how long an SMSC should attempt to deliver an SMS message. It is usually only useful when a destination recipient handset is turned off or out of coverage, and an SMS message is time critical. The value of the validity period is the “number of minutes” from the time the SMS message is submitted, until it should expire. The value may be specified as an absolute (date and time to the second) or relative (indicated in HHMMSS). When this time is reached, an SMSC would generally return an expired delivery report. If no validity period is set, the Sinch SMSC default is used which is 2 days (2,880 minutes). For example - setting the validity period to 30 would result in an SMS message only being retried for delivery by an SMSC for a maximum of 30 minutes.