Usually people tend to use strotime() indiscriminately, without being aware of its limitations. Although it is an awesome function, it needs to be used with care, otherwise it may lead to unexpected behaviors.
Below there’s a list of notes, also available in the PHP documentation, that everyone should keep in mind before using this function:
If the number of the year is specified in a two digit format, the values between 00-69 are mapped to 2000-2069 and 70-99 to 1970-1999. See the notes below for possible differences on 32bit systems (possible dates might end on 2038-01-19 03:14:07).
The valid range of a timestamp is typically from Fri, 13 Dec 1901 20:45:54 UTC to Tue, 19 Jan 2038 03:14:07 UTC. (These are the dates that correspond to the minimum and maximum values for a 32-bit signed integer.) Additionally, not all platforms support negative timestamps, therefore your date range may be limited to no earlier than the Unix epoch. This means that e.g. dates prior to Jan 1, 1970 will not work on Windows, some Linux distributions, and a few other operating systems. PHP 5.1.0 and newer versions overcome this limitation though. For 64-bit versions of PHP, the valid range of a timestamp is effectively infinite, as 64 bits can represent approximately 293 billion years in either direction.
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator between the various components: if the separator is a slash (/), then the American m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the European d-m-y format is assumed. To avoid potential ambiguity, it’s best to use ISO 8601 (YYYY-MM-DD) dates or DateTime::createFromFormat() when possible.
Returns a timestamp on success, FALSE otherwise. Previous to PHP 5.1.0, this function would return -1 on failure.