Timestamp Dependence

1. block.timestamp
   1. If the scale of your time-dependent event can vary by 15 seconds and maintain integrity, it is safe to use a block.timestamp.
2. block.number
   1. It is possible to estimate a time delta using the block.number property and average block time, however this is not future proof as block times may change (such as fork reorganisations and the difficulty bomb). In a sale spanning days, the 15-second rule allows one to achieve a more reliable estimate of time.