If you don't unit test from day 1 then there's a strong probability that you wont be writing your code in a way that you can unit test later. e.g. mockable depe

If you don't unit test from day 1 then there's a strong probability that you wont be writing your code in a way that you can unit test later. e.g. mockable dependencies. I'm included to say go with TDD right from the start.