clock: Fix DST problem in [clock scan] using "-gmt 0"

[prpr19xx]

Currently (in the UK, pre-DST, with "now" set to 1711500000):

. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 0   ;# Correct
1711843200
. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711843200

. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 0   ;# Wrong
1711929600
. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711929600

Next week (post-DST, with "now" set to 1712000000):

. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 0   ;# Wrong
1711839600
. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711843200

. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 0   ;# Correct
1711926000
. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711929600

This because tm.tm_isdst is set by localtime_r() using "now" set by time(NULL). It is nonsensical to specify a date to convert and have the DST flag used in the conversion set from another (i.e. the current) date.

With the patch (set tm.tm_isdst to -1 and leave mktime() to figure it out), it is correct under all circumstances:

. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 0   ;# Correct
1711843200
. clock scan {2024-03-31 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711843200

. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 0   ;# Correct
1711926000
. clock scan {2024-04-01 00:00:00} -format {%F %T} -gmt 1   ;# Correct
1711929600

For reference:

. clock format 1711839600 -format {%F %T %Z} -gmt 1
2024-03-30 23:00:00 GMT

. clock format 1711843200 -format {%F %T %Z} -gmt 1
2024-03-31 00:00:00 GMT

. clock format 1711926000 -format {%F %T %Z} -gmt 1
2024-03-31 23:00:00 GMT

. clock format 1711929600 -format {%F %T %Z} -gmt 1
2024-04-01 00:00:00 GMT

[msteveb]

LGTM. Thx