strtotime translates a date presented in various formats into a unix timestamp. It allows the syntax "+2 weeks" and various other time units to push that chosen date translation into the future. In 4.3.10, php would allow a space between the + and the beginning of the parameter. that doesn't seem to be the case any more. ("+3 days", not "+ 3 days"). Other than that, pretty smooth changeover so far.
It's pretty subtle, but I could potentially cause some headaches in the future - so keep an eye out!