GoogleCalendar Datekey

Question

Google is using an unique DateKeys for each days in the GoogleCalendar HTML e.g.

<div 
  data-datekey="129"
  role="gridcell" 
  tabindex="-1" 
  jsname="RjPD4e" 
  aria-labelledby="tsc-0" 
  data-column-index="0"  
  data-principal-ids="amFuLm5pY2tsYXNAbmFtaWNzLmNvbQ" 
  class="YvjgZe Qbfsob">

Is there any formular to calculate the date for a given datekey?

Answer 1

In response to @jantimon's answer: I guess they had to do a lot of calculations with dates and wanted it to be efficient.

The odd thing is that if you look at the little calendar in the sidebar, rather than using a data-datekey attribute, it uses data-date with a YYYYMMDD format.

Anyway, I rewrote your conversion function using bitwise operations, since it's easier to read this way.

function datekeyToDate(key) {
    /* # BITS: [year_rel(6)][month(4)][day(5)] */
    const day = key & 0b11111;
    const month = (key>>5) & 0b1111;
    const year_rel = (key>>9);
    const year = 1970 + year_rel;
    return new Date(year, month, day);
}

function dateToDatekey(date) {
    const y = date.getFullYear() - 1970;
    const m = date.getMonth()+1;    /* getMonth() returns 0-based index */
    const d = date.getDate();
    return (y<<9) + (m<<5) + d;
}