Timeline: Key dates in Greece's funding talks with euro zone

Athens also needs to negotiate by the end of June a longer-term financing arrangement with its lenders, because it is effectively cut off from financial markets and will default without money from the euro zone and IMF.

To clinch a deal, Greek negotiators have to agree on the comprehensive list of reforms with representatives of the European Commission, the IMF, the European Central Bank and the European Stability Mechanism (ESM) -- called "the institutions".

Once there is a deal with them, it can be presented to euro zone deputy finance ministers - the Eurogroup Working Group - and, with their approval, to finance ministers in the Eurogroup.

With Eurogroup backing for the plan, the ECB can again allow Greek banks to use Greek government bonds as collateral in ECB refinancing operations, which would solve the most immediate funding problems.

When reform legislation goes through parliament, euro zone finance ministers can instruct the ESM bailout fund to begin disbursing the remainder of euro zone aid - 1.8 billion euros. The IMF could also then release some of the 3.5 billion euros remaining in its bailout, which expires in March 2016.

Greece would then also be eligible to receive 1.9 billion euros in profits the ECB made by buying Greek bonds since 2010.

Below are key dates for Greece in the negotiations:

April 23 -- German Chancellor Angela Merkel meets Greek Prime Ministers Alexis Tsipras on the sidelines of an EU summit. No breakthrough expected as euro zone negotiators lack financial data withheld by Greece.

April 24 -- Euro zone finance ministers meet in Riga. They are expected to take stock of the insufficient progress in the talks and give guidance on how to proceed.

April 29 -- Weekly ECB discussion on the size and terms of Emergency Liquidity Assistance (ELA) for Greek banks. The ECB holds these reviews weekly until further notice.

April 30 -- Deadline for a deal on a comprehensive package of reforms agreed between Greece and its creditors on Feb 20. Missing the deadline has no practical implications except political embarrassment.

May 1 -- Greece has to pay back 200 million euros of interest to the IMF, but can delay the transfer until May 4 or 5 due to the May Day public holiday long weekend.

May 6 -- Non-monetary policy meeting of the ECB Governing Council.

May 8 -- Greece has to roll over 1.4 billion euros worth of maturing 6-month Treasury bills.

May 11 - Meeting of euro zone finance ministers in Brussels. Sever senior officials have pointed to that meeting as crucial because Greece may not have enough cash to make a big repayment to the IMF the next day. Greece said on April 22 it would have enough money for all of May thanks to a decree ordering state-owned companies to keep idle cash in the central bank.

May 12 - Greece has to pay back around 750 million euros of principal to the IMF.

May 15 - Greece has to roll over 1.4 billion euros worth of maturing 3-month T-bills.

May 20 - Non-monetary policy meeting of the ECB Governing Council.

End of May - Greece has to pay about 2.5 billion euros in salaries and pensions.

June 3 - ECB Governing Council monetary policy meeting.

June 17 - Non-monetary policy meeting of the ECB Governing Council.

June 18 - Meeting of euro zone finance ministers in Luxembourg.

June 25/26 - European Union leaders hold summit in Brussels.

June 30 - Expiry of extended bailout agreement between the euro zone and Greece, which means the end of access to the funds left over from the euro zone bailout. From then on, to get any money from the euro zone, Greece would have to negotiate a new bailout agreement from scratch.

(Reporting by Lefteris Papadimas in Athens and Jan Strupczewski in Brussels; Editing by Paul Taylor)