localtimes.js

const iso8601RegEx = /([0-9]{4})-([0-1][0-9])-([0-3][0-9])T([0-2][0-9]):([0-5][0-9])Z/

function format2Digits(num) {
    let asStr = '0' + num.toString()
    return asStr.slice(-2)
}

function EntryTimestamp(node) {
    this.node = node
    this.date = function() {
        let iso8601 = node.title
        let timeFields = iso8601.match(iso8601RegEx)
        let timestamp
        if (timeFields != null) {
            timestamp = Date.UTC(timeFields[1], timeFields[2]-1, timeFields[3],
                                 timeFields[4], timeFields[5])
        } else {
            timestamp = Date.parse(iso8601)
        }
        return new Date(timestamp)
    }()

    if (Intl == undefined) {
        this.dayFormatter = {
            format: function(date) {
                return date.getDate()
            }
        }
        this.monthFormatter = {
            format: function(date) {
                return ['Jan', 'Feb', 'Mar',
                        'Apr', 'May', 'Jun',
                        'Jul', 'Aug', 'Sep',
                        'Oct', 'Nov', 'Dec'][date.getMonth()]
            }
        }
        this.yearFormatter = {
            format: function(date) {
                return date.getFullYear()
            }
        }
        this.timeFormatter = {
            format: function(date) {
                let hours = format2Digits(date.getHours())
                let mins = format2Digits(date.getMinutes())
                return `${hours}:${mins}`
            }
        }
    } else {
        this.dayFormatter = new Intl.DateTimeFormat([], {
            day: "numeric"
        })
        this.monthFormatter = new Intl.DateTimeFormat([], {
            month: "short"
        })
        this.yearFormatter = new Intl.DateTimeFormat([], {
            year: "numeric"
        })
        this.timeFormatter = new Intl.DateTimeFormat([], {
            hour: "numeric",
            minute: "numeric",
            hour12: false
        })
    }

    this.getFieldText = function(fieldClass) {
        switch (fieldClass) {
            case "day":
                return this.dayFormatter.format(this.date)
            case "mo":
                return this.monthFormatter.format(this.date)
            case "yr":
                return this.yearFormatter.format(this.date)
            case "time":
                return this.timeFormatter.format(this.date)
        }
    }

    this.localtimeNode = function() {
        let node = this.node.cloneNode(true)
        for (let i in node.children) {
            let c = node.children[i]
            let t = this.getFieldText(c.className)
            if (t != undefined) {
                let text = document.createTextNode(t)
                c.replaceChild(text, c.firstChild)
            }
        }
        return node
    }

    this.updateDisplay = function() {
        let newNode = this.localtimeNode()
        this.node.parentNode.replaceChild(newNode, node)
        this.node = newNode
    }
}

function* getEntryTimestamps() {
    let dts = document.getElementsByClassName('datetime')

    for (let dt of dts) {
        for (let node of dt.getElementsByClassName('iso8601')) {
            if (node.getElementsByClassName('time').length > 0) {
                yield new EntryTimestamp(node)
            }
        }
    }
}

function switchAllEntryTimestampsToLocalTime() {
    for (let timestamp of getEntryTimestamps()) {
        timestamp.updateDisplay()
    }
}

switchAllEntryTimestampsToLocalTime()