TimeJump | TimeJump – Deep-links for Podcasts

Depending on your exact time zone (and jurisdiction), at some time in the early morning on 2018-10-28, the clock is moved back 1 hour because DST ends. From your examples, it seems that happens at 3:00 in your timezone / jurisdiction.

In the first case (adding 7200 seconds to 2018-10-28, 1:52:16), the tm_sec value is outside of the normal range (0 - 59), so mktime can determine that you've added 2 hours, and since it knows that that's traversing the DST boundary, it adjusts the time accordingly. This results in 2018-10-28, 2:52:16, which is 2 hours after 2018-10-28, 1:52:16.

For the next increment in the first case (adding 7200 seconds to 2018-10-28, 2:52:16), the exact same thing happens again (since you're again traversing the DST boundary - you have reset tm_isdst to -1 after all). This results in 2018-10-28, 3:52:16, which is 2 hours after 2018-10-28, 2:52:16.

In the second case (adding 2 hours to 2018-10-28, 1:52:16), the tm_hour value is still inside of the normal range (0 - 23), so mktime cannot determine that you've added 2 hours, and it just treats it like a local time. This results in 2018-10-28, 3:52:16, which is 3 hours after 2018-10-28, 1:52:16.

To avoid issues like these :

• do not reset tm_isdst to -1 unless needed (and you understand what will happen)
• work as much as possible with UTC timestamps, and only convert to local time when displaying.