RRSP contributions in first 60 days of calendar year

You can only claim up to your RRSP limits in a year. So if you have maxed out your RRSP in 2020 then you can't claim in 2020 any contributions made in the first 60 days of 2021. You can claim them in tax year 2021.

If you claim them in the previous year then the next year's limits will be adjusted to reflect the contribution, i.e. will be lower by the amount you claimed of your first sixty days contribution.

EXAMPLE

Let's say you are earning enough so that you are due $20,000 RRSP contribution room every year. You contribute $18,000 in 2020.

On 1st January 2021 you can contribute up to $22,000 ($2000 of contribution room left over from 2020 and $20,000 because of what you earned in 2020). You can claim $2000 of that on your 2020 tax form i.e. up to your 2020 maximum. The rest you can claim on your 2021 tax form.