SLA29970'S PROFILE

Look at the IANA tz data for Regina, in the file northamerica, these lines
# Zone NAME GMTOFF RULES FORMAT
Zone America/Regina -6:58:36 - LMT 1905 Sep
-7:00 Regina M%sT 1960 Apr lastSun 2:00
-6:00 - CST
this says that as of 1905 September the city of Regina is deemed to have switched from using the mean solar time of its meridian of longitude to using Mountain Time.