Starting from creation.
First Bhagavan makes Pancha-Bhootas (5 elements) from Prakriti (akasha/ether).
Then he gives life to Brahma. Then Brahma starts creation.
Lifetime of Brahma = 100 'brahma-years'. (not human years, as you'll see shortly)
1 Brahma-year = 360 'brahma-days'.
1 Brahma-day = 1 daytime + 1 nighttime
1 Brahma-daytime = 1000 chatur-yuga (4-yuga or MahaYuga)
1 Chatur-Yuga = Krita + Treta + Dwapara + Kali Yugas
Kali Yuga = 100 + 1000 + 100 = 1200 'deva-years'
Dwapara Yuga = 200 + 2000 + 200 = 2400 'deva-years'
Treta Yuga = 300 + 3000 + 300 = 3600 'deva-years'
Satya/Krita Yuga = 400 + 4000 + 400 = 4800 'deva-years'
Total Chatur-Yuga = 12,000 'deva-years'
1 Deva-year = 360 'deva-days'
1 Deva-day = 1 daytime + 1 nighttime
1 Deva-daytime = 6 'human-months' (Uttarayana mid-Jan to mid-July)
1 Deva-nighttime= 6 'human-months' (Dakshinayana mid-July to mid-Jan)
1 Deva-day = 1 'human-year'
1 Human-year (or just year) = 12 months
1 month = 2 fortnights (Krishna Paksha & Shukla Paksha)
1 fortnight = 2 weeks
1 week = 7 days
1 day = 60 nadis
1 nadi = 60 vinadis (or 24 minutes)
1 vinadi = 24 seconds
Thus 1 day = 60nadis x 24minutes. While 'modern' day is 24hours x 60minutes.
The years follow a 60-year cycle.
The seasons follow 6-season cycle - 2 months per season
The months follow a 12-month cycle
http://veda.wikidot.com/vedic-time-system