Each player skips their untap step.
At the beginning of each player's upkeep, that player simultaneously untaps each tapped artifact, creature, and land they control and taps each untapped artifact, creature, and land they control.
You can tap lands for mana before Sands of Time toggles your permanents. The mana you gain is available during your upkeep step, but it will disappear before your draw step if you don't spend it. You cannot save it to be spent on the card you will draw this turn.