If you are paying quarterly taxes as an independent contractor and you work in March but get paid for that work in April or May, when do you owe taxes for the work in March?
This is in the US.
Chances are, you’re a cash-basis taxpayer, so income is considered earned when you actually receive it.
So, you’re looking at the estimated taxes being due in your Q2 payment.
(Of course, my general recommendation with estimated payments is to make each one equal to a quarter of last year’s tax due. This avoids any under-withholding penalties without having to worry about this year’s income or the variations from one quarter to the next. If tax is actually higher this year than last year, just save the difference and pay it when you file the return.)