Fix timezone issues by enforcing UTC for all time operations

[Copilot]

Fix timezone issues with time.Now() usage

Based on analysis of the codebase, I've identified and fixed all instances of time.Now() to use UTC timezone to avoid issues during time shifts.

Changes completed:

• Analyze all time.Now() usages and categorize by type
• Fix timing/latency measurements in modes.go (17 instances)
• Fix result tracking in pkg/results/ (5 instances)
• Fix workload generators in pkg/workloads/ (4 instances)
• Fix random seed generation in random/random.go (1 instance)
• Fix main.go startTime initialization (1 instance)
• Update test files appropriately (4 instances)
• Write comprehensive tests to validate UTC enforcement
• Run full test suite to verify changes (all tests pass)
• Validate with build and manual testing
• Fix linting error (variable shadowing)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Timezone issue</issue_title>
<issue_description>I see that code uses time.Now() everywhere it leads to the problem when you run test during time shift.
It would be better to, at least internally, enforce UTC timezone.

Migrated to Jira: QATOOLS-70

<agent_instructions>suggest fixes for all of those usage of time.Now()</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@dani-tweig Closing this issue as it was moved to Jira. Please continue the thread in https://scylladb.atlassian.net/browse/QATOOLS-70

• Fixes Timezone issue #222

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[fruch]

@copilot

linting is faliing:
https://github.com/scylladb/scylla-bench/actions/runs/21109802242/job/60708164253?pr=244

[Copilot]

@copilot

linting is faliing:
...

Fixed in commit 81a2fe8. The issue was a variable shadowing warning - the local variable startTime in the test was shadowing the global startTime variable from main.go. Renamed it to histogramStartTime to resolve the govet shadow warning.

[CodeLieutenant]

@fruch I've already worked on it 2 days ago, haven't finished it, will push it in tomorrow.
Mine implementation is little bit different. I've extracted time.Now() into separate file and created a function around it, so that if we need to change it to some different timezone for some reason it's only in one place, and not scattered like it's now

[fruch]

@fruch I've already worked on it 2 days ago, haven't finished it, will push it in tomorrow. Mine implementation is little bit different. I've extracted time.Now() into separate file and created a function around it, so that if we need to change it to some different timezone for some reason it's only in one place, and not scattered like it's now

it's a basic builtin utility, not sure wrapping it is a better solution.

also if you were working on something, you should state it on the issue (move it in progress by the minimum)