The only logic I can apply is. These are the point of perspective of higher beings viz tridevs from their owm calculation of time.
Sri Shiva Purana 2.1:1:10.:
15. Listen to the span of life of the deities—Brahmā, Viṣṇu and Hara. There shall not be any doubt in this respect.
16. A thousand sets of the four-yuga periods constitute one day of Brahmā. The period of night is also similar. Further measurement of time is based on this calculation.
17. Thirty such days (days and nights) constitute one month and twelve months, one year. The span of life of Brahmā is hundred such years.
18. One year of Brahmā constitutes one day of Viṣṇu. Viṣṇu lives for hundred years in his own calculation.
19. One year of Viṣṇu constitutes one day of Rudra. When a hundred year period passes, Rudra assumes the form of Nara (supreme Man).
20. He (Rudra) stays like that as long as the breath is retained by Sadāśiva. When He (Sadāsiva) exhales he (Rudra) merges into Śakti (Prakriti).
21-22. In the case of all living beings, Brahmā, Viṣṇu, Hara, Gandharvas, serpents, Rākṣasas, etc., twenty one thousand six hundred respirations constitute the period of one day and one night, O foremost among Devas.
23-24. Six respirations constitute the period of time one Pala. Sixty such Palas constitute one Ghaṭī. Sixty Ghaṭīs constitute one day and one night. (6 x 60 x 60 = 21600). There is no limit to the number of respirations of Sadāśiva. Hence He is undecaying.
Thus, everyone have their each sets of time cycle.
So, here to my understanding, Lord Brahma is saying he took 6 births from Lord Vishnu already and this is his 7th birth.
- 1 Brahma = 100 days of Vishnu.
- 6 Brahmas = 600 days of Vishnu.
- 1 month = 30 days.
- 6 Brahmas = 20 months of Vishnu.
- 7th Brahma's age = 51 years = 20 months + 1 month + 21 days of Vishnu.
Thus, we are in the 21st day of the 21st month (1 year and 9 months) of Lord Vishnu.
Similarly, we are in the
18th hour of the 2nd day of Lord Rudra. As for Bhagawan Sada Shiva, he might have just started inhaling.
Even Sri Skanda Purana 188.8.131.52 also confirms this.:
Shiva Said -
Six Brahmās have passed away. The present Prajāpati who is well-known as Śatānanda is the seventh one, O goddess.
Conclusion.: It's the same Brahma who takes birth from Lord Vishnu in various ways and this is his 7th birth and we are in his 51st year.
Bdw, That's just my theory.
I hope this clarifies your queries.