##### Ambiguous Times

Ambiguous times are where, for any given time, there are at least two correct alternatives. This situation occurs when converting a local time to a UTC time at a time when the clocks are being turned back to standard time from daylight savings time

Take the following situation as an example:
The local time is established as 2008-10-26 01:23:54 and you wish to convert this time to UTC.
In the UK the standard time is GMT which is in effect the same as UTC. Daylight savings time (known as British Summer Time) is GMT + 1 hour. The time the change from BST to GMT is made is at 02:00:00 (BST) on the last Sunday in October. In effect, at 2am on that Sunday the clocks go back to 1am. The hour between 1am and 2am therefore occurs twice.
The problem in converting this given local time is that it occurs at this changeover and there are two legitimate UTC times:
2008-10-26 00:23:54 UTC when the local clock was GMT (UTC) +1 hour and
2008-10-26 01:23:54 UTC when the local clock was the same as UTC.

All time zones that apply DST are subject to this effect. In such a situation a given local time is said to be 'ambiguous'.