# [Datetime-SIG] Calendar vs timespan calculations...

Ethan Furman ethan at stoneleaf.us
Sat Aug 1 19:52:29 CEST 2015

```On 08/01/2015 08:27 AM, Alexander Belopolsky wrote:

> In the case of the US-style spring jump from 01:59 to 03:00 AM, for t
> = 02:30 AM, u0 is such that L(u0) = 03:30 AM (this is the "what a
> meant when I said 02:30" time) and L(u1) = 01:30 AM.

The problem here is that if somebody is counting backwards to get that 1:30, then the time they need is 12:30, not 2:30.

As a case in point:  Today I have veterinary appointment for my cat to check his medication levels; the appointment is at 14:30, and needs to be in the window of 4 to 6 hours of him taking his meds.
Counting backwards from 14:30 gives me a window of 8:30 to 10:30 to administer his meds.

I'm happy to concede that counting backwards to get the start time is less frequent, and the times this hits/crosses a time shift are even less frequent, but that is all the more reason to refuse the
temptation to guess.

--
~Ethan~
```