Does a continuous effect that sets a creature's power to zero override a pump spell applied afterward?

Continuous effects that modify power and toughness are applied in a specific sequence called the "layer system," specifically within Layer 7, which is subdivided into 7a (characteristic-defining abilities), 7b (effects that set power/toughness to a specific value), 7c (effects that modify power/toughness by a fixed amount), and 7d (effects that switch power and toughness). CR 613.3 governs this ordering.

An effect that sets a creature's power to 0 — such as a static ability or a spell like Diminish — applies in Layer 7b. A pump spell like Giant Growth (+3/+3) applies in Layer 7c. Because 7c is always applied after 7b regardless of timestamps, the +3 bonus is added on top of the 0. The creature ends up with 3 power, not 0. CR 613.3c and 613.3d make this explicit.

The timestamp rule (CR 613.7) is used to break ties only when two effects occupy the same sublayer. Since a set-to-value effect (7b) and a modify-by-amount effect (7c) are in different sublayers, timestamps are irrelevant here — sublayer order always wins.

Concrete example: Your opponent controls an enchantment that sets your Grizzly Bears' power to 0. You cast Giant Growth (+3/+3) targeting it. Even though the enchantment was in play first, the +3 from Giant Growth is applied in 7c after the 0-setting in 7b, so your Bears become a 3/5 until end of turn.

The key takeaway: pump spells that add a fixed amount are never "overridden" by set-to-zero effects applied earlier or even later, because sublayer 7c always resolves after sublayer 7b. Only another 7b effect with a later timestamp could override the zero-setter.