Depends the nature of the weather. If it is removing an operational requiremnt ( ie holding fuel or alternate), it ceases 30 min after the end of its time frame ( ie if the bad weather stopped at 0200, the requirement is there until 0230).
If it is creating an operational requirement ( ie the weather was good but will soon turn to ****), the requirement comes in 30 min before its forecast time. For example if there was a FM 0200 of bad weather below alternate minima, the requirement is applicable and must be accounted for from 0130.
However, consider an example when the weather was below alternate minima and your ETA at the AD is 0210z, however a FM period from 0200z lifted the weather above the alternate minima. Since it is removing an operational requirement, the buffer period pushes this requirement out until 0230z. However, you dont need to plan an alternate even though weather is below minima, you just need holding fuel until the requirement is removed, ie 20 min.