or PT1400M or P0.5DT700M?

oh, you can use commas too.

and if you're still not thinking this is fun, here's a quote from Wikipedia "But keep in mind that "PT36H" is not the same as "P1DT12H" when switching from or to Daylight saving time."

just add a unit to your period parameters. sigh.

Maybe surprising for days (could also happen with minutes because of leap seconds technically :-) but for months and years this is more apparent due to month ends and leap years.