They also had to deal with poor weather last year, when at the peak period of summer 85 days of work were lost.
The interior of the building is almost complete and the granite walls are covering more of the metal structure, but the authority said it was impossible at present to confirm a completion date.
Speaking at the construction site, Aneurin Phillips, chief executive of Snowdonia National Park Authority said: "The granite walls need to be completed, the building will need to be commissioned and crucially the railway track at the summit needs to be re-laid, all of which will take time.
"However, the more I see of the work the more I am convinced that this will be a magnificent building for the summit of this beautiful and at times challenging mountain."
The building, which is called Hafod Eryri, needs to be completed before the autumn deadline to draw on funds from Europe.
The assembly government has supported the project and backing has also come from European Objective One funds. A public appeal also raised £350,000.
Refreshments and shelter
Hafod Eryri will replace the demolished mountain cafe, which was once called Britain's highest slum by Prince Charles.
The old summit building was built in 1935 and designed by Sir Clough Williams-Ellis, the founder of the Italianate village Portmeirion a few miles away on the north Wales coast.
The centre will have facilities giving visitors the chance to learn more about Wales' highest mountain and its environment.
It will also provide weather information, advice on routes as well as refreshments and shelter
The building's name was chosen after a secret ballot. Eryri means Snowdonia in Welsh, but there is no direct translation for hafod, an old Welsh term for residence on high land.
Meirion Evans, regional director of construction company Carillion, said: "We have had to face problems and conditions on this scheme which are as unique as the location itself.
"We have found solutions and worked extremely hard every moment the mountain and conditions will allow.
"I am aware that people will be disappointed at the delays which mean that plans cannot be laid with certainty for opening Hafod Eryri to the public but we could never expect the mountain to give us free rein," he added.
"We ask everyone to be patient while we work with the conditions and I know that we will deliver a building of which everyone can be proud".
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