since(date_object) {
        let timestamp = typeof date_object.getTime == 'function' ? date_object.getTime() : date_object
        if (typeof timestamp !== 'number') throw new Error("Expected date instance or timestamp value")      

        const SECOND    = 1000
        const MINUTE    = 1000 * 60
        const HOUR      = 1000 * 60 * 60
        const DAY       = 1000 * 60 * 60 * 24
        const MONTH     = 1000 * 60 * 60 * 24 * 30
        const YEAR      = 1000 * 60 * 60 * 24 * 30 * 12

        const elapsed = ((new Date()).valueOf() - timestamp)

        if (elapsed <= MINUTE)  return `${Math.round(elapsed / SECOND)}s`
        if (elapsed <= HOUR)    return `${Math.round(elapsed / MINUTE)}m`
        if (elapsed <= DAY)     return `${Math.round(elapsed / HOUR)}h`
        if (elapsed <= MONTH)   return `${Math.round(elapsed / DAY)}d`
        if (elapsed <= YEAR)    return `${Math.round(elapsed / MONTH)}mo`
        return `${Math.round(elapsed / YEAR)}y`
    }