Compare ‘ Will have + past participle ‘with other perfect forms.  This is a short comparison of ‘ Present Perfect, Future Perfect and Past Perfect ‘

Examples:

• They have been married for 20 years.( present perfect – shows the present position. We don’t say , ‘ They are married for…………..))
• By the end of December next year they will have been married for 21 years. ( future perfect   – shows the future position.  We don’t say, ‘ They will be married for……..))
• When their first child was born, they had been married for three years. ( past perfect – shows the past position.  We don’t say , ‘ They were married for……..))