Only half of children born in 2011 and last year will be guaranteed two years of nursery provision, according to research.
The current legal entitlement to Scottish Government-funded places can vary by up to 317 hours or £1,033 based on date of birth, think-tank Reform Scotland found.
It warns that changes in the Children and Young People Bill being discussed at Holyrood today will widen the gap to 400 hours or £1,305.
Alison Payne (pictured), research director of Reform Scotland, said: "These new figures will come as a shock to many people and the cause of the unfairness can be easily remedied. Parliament can right this wrong.
"The solution is simple: just as all children are entitled to seven years of primary education irrespective of their date of birth, they should be entitled to a basic two years of government-funded nursery provision.
"To achieve this, Reform Scotland believes that nursery provision should start at a fixed point in the year, probably in August, just as it does for school.
"Our solution would ensure that all children had a legal entitlement to two years' government-funded nursery provision and looked-after children would receive a full additional year's provision."
Reform Scotland's research showed a child born between March 1 and August 31 will be entitled to two years nursery provision before beginning school.
A child born between September 1 and December 31 gets 18 months and a child born between January 1 and February 28, assuming he or she attends school aged four, will get 15 months, according to the research.
A Scottish Government spokeswoman defended the system, saying it will save parents of three and four-year-olds, as well as some vulnerable two-year-olds, up to £700 a year.
"We are providing an extra £50 million to councils in 2014-15, as part of a total package of £190 million, to help them meet that extra demand," she said.
"The priority for the Scottish Government at this stage is to build additional hours and flexibility into high-quality childcare, increasing the entitlement to around 16 hours a week. This is a huge step forward in improving our young children's prospects and supporting parents.
"Some variation will remain whether a child begins nursery on their birthday or the following term, but this system currently makes the best use of the entitlement in relation to the child's age and needs with parents of younger children able to defer school if they feel it best."
Conservative MSP Liz Smith backed calls for a fixed starting point.
"Reform Scotland has exposed the truth that the so-called guarantee doesn't do what it says on the tin," she said.
"Parents up and down the country will rightly wonder how it can be that children born only days apart can receive such varying levels of nursery care."
Liberal Democrat MSP Liam McArthur said the Government should show more ambition, particularly for two-year-olds from the poorest backgrounds.
"Children 1st and others have warned there is a risk that children in Scotland could fall behind their counterparts south of the border unless improvements are made to the Bill," he said.
"Alex Salmond promises this will all change with independence. A growing chorus of voices, however, are demanding action on better childcare now and not on the basis of how they vote in a referendum in the future."