"Year one thousand nine hundred and sixty one"

Translation: Mwaka elfu moja mia tisa sitini na moja

[Juswam]

Mwaka "wa" elfu moja...

[KathyKerns]

What does the "wa" mean in this sentence?

[DynastyRoyale]

Can a na be added between tisa and sitini since it's nine hundred and sixty one?