Does Zhonya save temporary unit from its death once?

Yes, Zhonya's Hourglass will save a unit with the Temporary keyword from dying, but only for that specific turn.

According to the FAQ and Rule 366.1:

• Zhonya's Hourglass is a replacement effect that alters the execution of a "die" event. When the Temporary trigger attempts to kill the unit at the start of the beginning phase, Zhonya's replaces that death with a recall effect.
• The unit is recalled to base and exhausted, but it retains the Temporary keyword.
• Because the unit still has the Temporary keyword, it will die again at the start of the next turn's beginning phase when the Temporary trigger occurs again.

Sequence:

1. At the start of the beginning phase, the Temporary trigger is added to the chain to kill the unit.
2. Zhonya's Hourglass (a replacement effect) intercedes, replacing the "die" instruction with "heal, exhaust, and recall."
3. The unit is saved for the current turn.
4. At the start of the next turn's beginning phase, the Temporary trigger occurs again, and the unit will die unless another effect saves it.

*Note: This is based on the FAQ entry: "Zhonya's Hourglass will protect the unit for the current turn by replacing its death with recall, but the unit retains the Temporary keyword and will die at the beginning of the next turn."