use AppleScript version "2.4" — Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

set tmpDate to "2014-09-14 15:45:00"
set tmpD1 to dateFromStringWithDateFormat(tmpDate, "yyyy-MM-dd HH:mm:ss") of me
–> date "2014年9月14日日曜日 15:45:00"

set tmpDate to "August 19, 2018 15:45:00"
set tmpD2 to dateFromStringWithDateFormat(tmpDate, "MMMM dd, yyyy HH:mm:ss") of me
–> date "2018年8月19日日曜日 15:45:00"

return {tmpD1, tmpD2}

on dateFromStringWithDateFormat(dateString, dateFormat)
　　set dStr to current application’s NSString’s stringWithString:dateString
　　set dateFormatStr to current application’s NSString’s stringWithString:dateFormat
　　
　　set aDateFormatter to current application’s NSDateFormatter’s alloc()’s init()
　　aDateFormatter’s setDateFormat:dateFormatStr
　　aDateFormatter’s setLocale:(current application’s NSLocale’s alloc()’s initWithLocaleIdentifier:"en_US_POSIX")
　　
　　set aDestDate to (aDateFormatter’s dateFromString:dStr)
　　
　　return aDestDate as list of string or string
end dateFromStringWithDateFormat