This depends on the way the platform mktime implements folding when the clock is advanced for daylight saving time. It appears the timezone on your systems is set to US/Canada Central Time, for which on March 9th the clocks were advanced forward at 02:00:00 [1]. Windows mktime folds back at 02:00:00, repeating the 01:00:00-01:59:59 timestamps. Linux mktime has a fold at 03:00:00, repeating the 02:00:00-02:59:59 timestamps. What matters is that both are correct on the boundaries for observed local times, i.e. they both give the same values for 01:59:59 and 03:00:00, which are respectively 1394351999 and 1394352000.

[1]: https://www.timeanddate.com/time/dst/2014a.html