WASHINGTON, Dec 3 (Reuters) - The U.S. government could fall behind on its bills this month - and even default on its debt - if Congress doesn"t raise a $28.9 trillion cap on government borrowing, a failure that could trigger economic calamity and panic on global financial markets. In years past, Congress has come extremely close to the edge of the default cliff, but has yet to actually fall off it. What follows is a timeline showing how a cascade of missed payments could unfold and their potential impact if lawmakers do not act on time, based on a scenario laid out by the Bipartisan Policy Center, a Washington-based think tank. DEC. 15The Treasury Department finishes transferring $118 billion into a federal highway fund as part of a $1 trillion bipartisan infrastructure law championed by Democratic President Joe Biden, who signed the legislation on Nov. 15. This puts the government dangerously close to its legal debt limit. Treasury Secretary Janet Yellen has already warned Congress her agency might not be able to pay all the government"s bills after this day. Should quarterly corporate tax receipts due Dec. 15 disappoint, the "X date" - the day when the Treasury has to start missing payments - could fall early in a range of possible dates projected by the policy center. DEC. 21 The Treasury"s cash coffers run dry and it hits the debt ceiling. The day"s $13 billion in tax revenues aren"t enough to cover $19 billion in spending obligations promised by Congress. Who doesn"t get paid? Possibly everybody expecting a check. When Washington brushed up against the debt ceiling in 2011, officials readied a plan to pay none of their bills on time rather than picking and choosing, according to an internal Treasury watchdog. Treasury would wait until it gets enough money to pay a full day"s bills before paying any of them. Lots of checks would not go out on Dec. 21, including $2 billion owed to defense companies and $5 billion for states to help pay medical bills for low-income Americans. DEC. 22 Spooked by the missed payments, stock markets could swoon. That could put pressure on Senate Republican leader Mitch McConnell, whose minority party can slow most legislation, to help Democrats quickly raise the debt limit. "McConnell does not want to go down in history books as the guy who cratered the world economy," said Philip Wallach, a political scientist at the conservative American Enterprise Institute. Without a deal, the Treasury would likely be able to cut the checks due Dec. 21 one day late. But then even bigger bills start coming due. On Dec. 22, $20 billion in payments don"t go out to pensioners and other Social Security beneficiaries - just days before the Dec. 25 Christmas holiday. Another $7 billion in health insurance subsidies go unpaid. The Treasury ends the day $32 billion behind on its bills. DEC. 23 The pile of unpaid obligations grows as the Treasury fails to pay $4 billion in federal salaries. States don"t receive the $2 billion they are owed for Medicaid health insurance subsidies for the poor. DEC. 27-DEC. 30 The Treasury gets some respite on Dec. 27 after taking in $31 billion in tax revenue. It can finally pay the Social Security benefits due several days earlier. But by Dec. 30, the Treasury doesn"t have money to pay $4 billion owed to active-duty soldiers and $11 billion for military veterans. DEC. 31 Things get really dicey on New Year"s Eve when the Treasury is due to pay investors about $10 billion in interest payments on the national debt. The Treasury said in 2014 - following another near-collision with the debt ceiling - that it is technically capable of prioritizing interest payments over other obligations. That"s important because missing an interest payment - which would put America in default on its debt - would rock the global financial system. "It would be uncharted waters," said Shai Akabas, the Bipartisan Policy Center"s director of economic policy.