You're seeing just the function to_timestamp-exclamation-mark, go back to SMPPEX.Pdu.ValidityPeriod module for more information.
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to_timestamp!(validity_period, timestamp_origin \\ System.system_time(:second))

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to_timestamp!(validity_period(), timestamp_origin()) :: timestamp()

Converts validity_period/0 to Unix timestamp according to the SMPP specification. The same as to_timestamp/2 but raises an exception in case of error.