Recently a user saw his estimated time fluctuating and thought it was a bug due to daylight savings time. Let me start by saying it is an impossibility of daylight saving time to affect your staking time in any way. This is because when calculating time, Reddcoin and most other systems use Unix Time, It calculates time from a point and changing a clock does not affect that.
Now on to Estimated time to stake and why it cant be an exact time. Estimated time is calculated by taking the network weight and dividing it by your total coin weight with a few other calculations to get hours.
These are the calculations Simplified without rounding
network weight = latest difficulty * 4294967296 / 60
esttimestake = networkweight / coinweight / 60
The current reddcoin difficulty is around 262 at time of writing but the difficulty fluctuates - see here http://www.coinwarz.com/difficulty-charts/reddcoin-difficulty-chart
I have a wallet with a balance of 241201.11475035 and coin weight of 1532157.0973999768 so we use the calculations above
262 * 4294967296 / 60 / 1532157.0973999768 / 60 = 204 hours
now the same some with a different recent difficulty
362 * 4294967296 / 60 / 1532157.0973999768 / 60 = 281 hours
Remember your coins gain weight over time so as your coin weight increases your time will come down even if the difficulty stayed the same. As you can see difficulty can make a huge difference as it fluctuates and this is why its an estimated time, only if it was a static difficulty could the time be more accurately predicted.
Calculations are taken from my RddCompanion chrome extension.
If you have any questions please ask and I will be happy to answer them