No, it doesn't work like that - that would make the tax retrospective and very unfair.
I use an example of how it works
You are now receiving a gift of 100k say. You need to calculate the value of all prior gifts received since 5th Dec 1991 let's say 400K in 2006 and 50k in 2008.
Step 1 calculate tax on current gift of 100k.
Taxable value of current gift is 100-3 = 97k (you can receive 3k tax-free from a donor every year)
Total value of all gifts received 400+50+97=547k
Current threshold is 250 so taxable value is 547-250 = 297
Tax due at 30% = 89,100
Step 2 Do same calculation excluding the current gift ie the 100k
Total value of all gifts received to date is 400+50=450k
Current threshold is 250 so taxable value is 450-250 = 200
Tax due at 30% = 60,000
Subtract to find tax due on current gift
89,100-60,000 = 29,100
This is also 97k x 30% - but the correct to arrive at this is as above.
Hope that helps.