Does the time of day matter for pruning lilacs or forsythia?

No, time of day doesn’t matter for pruning lilacs or forsythia. Focus on the season: prune spring-flowering shrubs right after they finish blooming, not in fall or winter. Pruning in late winter or early spring will cut off this year’s flowers. Any cool-ish part of the day is fine, but avoid full scorching heat for your own comfort.