Daily Value at Risk
While companies will sometimes lose more money in a single day than their "daily value at risk", this should occur infrequently if the company is modeling its risk correctly. If the value at risk is calculated at a 99% confidence interval, losing more money than the daily value at risk should occur around 1 in 100 days, or about 3 or 4 days out of the year. However, firms calculate daily value at risk by modeling out past market behavior. Assumptions can vary between firms and flawed assumptions can lead to flawed results. Additionally, if markets begin behaving in ways they haven't behaved in the past, the historical assumptions underlying Value at Risk models may no longer apply.
Although the industry standard is to calculate average daily value at risk at a 99% confidence level, banks can calculate this number at difference confident intervals - for example, at a 95% confidence interval, which would result in a smaller value at risk - but less confidence in whether this decline will be exceeded on any given day. It is also possible to calculate Value at Risk over larger timeframes, such as weekly or monthly.