Donor-advised funds have been around for years, particularly at community foundations aiming to encourage giving to local causes. But they have become increasingly popular over the last decade with the growth of national funds affiliated with big financial services companies like Fidelity, Vanguard and Charles Schwab. Fidelity Charitable, for instance, received more than $4 billion in donations in 2016, topping United Way Worldwide, which Forbes has ranked as the largest nonprofit organization by private donations.

The funds are not without controversy. Some organizations are wary of them, because there is no firm requirement for the funds to actually pay out the money within a certain period.

And there are also fees for donors to consider. Fidelity Charitable, for instance, charges an annual administrative fee of at least $100, plus additional investment fees. A chart on its website says that a $10,000 balance would incur annual fees of about $127, compared with hypothetical gains of $190, based on a conservative investment.

But there is no doubting the funds’ popularity. The National Philanthropic Trust, which prepares an annual report tracking the funds, said there were nearly 285,000 donor-advised fund accounts in 2016, an increase of about 7 percent over the prior year. And according to an analysis by The Chronicle of Philanthropy, annual contributions to the largest 85 donor-advised funds nearly tripled in value from 2008 to 2013.

If you might be considering “bunch” investing in this way, here are answers to some common questions about donor-advised funds:

Q. When is the deadline for contributing to a donor-advised fund for a 2017 tax deduction?

The deadline is Dec. 31. But as a practical matter, it is wise to act earlier, depending on how the contribution is made, according to financial advisers and managers of donor-advised funds. If someone is establishing a fund affiliated with a company where they already do business, transferring funds is relatively simple. Fidelity Charitable, for instance, says assets can be transferred from Fidelity Investments over the phone or online as late as Dec. 31.

But if you plan to pay by check, keep in mind that Dec. 31 is a Sunday, so you will want to document that it was mailed on Dec. 30 to make sure your contribution qualifies for a 2017 tax deduction. The recommended date for some types of contributions has already passed at various donor-advised funds. Vanguard Charitable, for instance, had recommended that securities held outside of Vanguard be contributed by Dec. 15.

There is still time to use a donor-advised fund, but it’s best to act sooner to avoid last-minute snafus and make sure a gift is counted for 2017 for tax purposes. “I would not wait until the middle of next week,” said Howard Hook, a financial planner and principal at EKS Associates in Princeton, N.J.