出现deferred tax assets或liabilities的原因是accounting standard和tax authorities对于expense和revenue的规定存在差异。tax expense大于payment的时候就会产生deferred tax liabilities，否则就是assets。
Tax base
教学里没有讲得很明白tax base有什么用，怎么参与计算，只能大概总结为如下：
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$$ \text{temporary difference} = \text{carrying amount} - \text{tax base} $$
- tax base也是存量值
- 以折旧为例，总值10000的机器，自己采用的折旧额是25%（2500每年），税局采用的是50%（5000每年），那么第一年结束的carrying amount就是7500，tax base则是5000
- 以费用为例，一笔10000的费用，自己选择当年全部expense，税局只允许expense 25%，那么第一年结束的carrying amount就是0，tax base则是7500
In general, a company will recognize a deferred tax asset or liability when recovery/settlement of the carrying amount will affect future tax payments by either increasing or reducing the taxable profit.
注意，$\text{temporary difference} \times \text{tax rate}$ 才是deffered tax items，符号是正那就是DTL，负则是DTA。
Tax rate changes
如果税率发生变化，deffered tax items也会同向变化——税率上升，DTA和DTL都会增加。
例外
- Deferred tax assets and liabilities should not be recognized in cases that would arise from the initial recognition of an asset or liability in transactions that are not a business combination and when, at the time of the transaction, there is no impact on either accounting or taxable profit.
- 比如政府给予的优惠，在财报上可能减少carrying amount，但是不能确认
- Initial recognition of goodwill
对子公司、分公司、联营公司的投资以及在合资企业中的权益可能会导致合并财务报表与母公司财务报表中的暂时性差异，根据DTA还是DTL有不同确认条件：
- DTL只有以下两个条件同时满足才不会确认：
- 母公司能够控制暂时性差异未来转回的时间
- 该暂时性差异很可能在未来不会转回
- DTA只有以下两个条件同时满足才会确认：
- 该暂时性差异在未来会转回
- 存在足够的应纳税利润可以用来抵扣暂时性差异
unused tax losses
IFRS: allows the recognition of unused tax losses and tax credits only to the extent that it is probable that in the future there will be taxable income against which the unused tax losses and credits can be applied.
GAAP: a deferred tax asset is recognized in full but is then reduced by a valuation allowance if it is more likely than not that some or all of the deferred tax asset will not be realized.
The existence of tax losses may indicate that the entity cannot reasonably be expected to generate sufficient future taxable income.
Recognition and Measurement
虽然DTA和DTL是未来现金流，但是不能折现，并且应该根据tax rate变动进行adjust。
Deferred tax和income tax都必须在income statement上recognize除非是属于（或）：
- 直接计入equity
- 与企业合并相关的provision for deferred tax
DTA和DTL的carrying amount是需要时不时评估的（比如税率变化、DTA的recoverability变化等），并且在资产负债表日必须重新评估：
- If there is any doubt whether the deferral will be recovered, then the carrying amount should be reduced to the expected recoverable amount. 后续如果情况好转，the reduction may be reversed.
- 对于GAAP，因为是用valuation allowance来计算的，所以如果后续情况好转，这个reversal会导致DTA和operating income增加。
Current taxes payable or recoverable are based on the applicable tax rates on the balance sheet date of an entity; in contrast, deferred taxes should be measured at the tax rate that is expected to apply when the asset is realized or the liability settled.
Charged directly to equity
In general, IFRS and US GAAP require that the recognition of deferred tax liabilities and current income tax be treated similarly to the asset or liability that gave rise to the deferred tax liability or income tax based on accounting treatment. Should an item that gives rise to a deferred tax liability be taken directly to equity, the same should hold true for the resulting deferred tax. 比如PPE的revaluation。
Whenever it is determined that a deferred tax liability will not be reversed, an adjustment should be made to the liability. The deferred tax liability will be reduced and the amount by which it is reduced should be taken directly to equity. Any deferred taxes related to a business combination must also be recognized in equity.