fix(date): improve cross-platform date command compatibility

Fixes date command errors on macOS with Homebrew GNU coreutils:
- 'date: invalid option -- v' when GNU date runs BSD syntax
- 'syntax error in expression' when stat returns filesystem metadata

Changes:
- Replace uname-based detection with capability detection (try/fallback)
- Try GNU commands first, fall back to BSD, then ultimate fallbacks
- Add date -r fallback for file mtime (most portable)

This handles mixed environments where uname returns "Darwin" but
GNU coreutils are in PATH from Homebrew.

Credit: @farce1 (PR #119)

lib/date_utils.sh

@@ -5,33 +5,33 @@
 
 # Get current timestamp in ISO 8601 format with seconds precision
 # Returns: YYYY-MM-DDTHH:MM:SS+00:00 format
+# Uses capability detection instead of uname to handle macOS with Homebrew coreutils
 get_iso_timestamp() {
-    local os_type
-    os_type=$(uname)
-
-    if [[ "$os_type" == "Darwin" ]]; then
-        # macOS (BSD date)
-        # Use manual formatting and add colon to timezone offset
-        date -u +"%Y-%m-%dT%H:%M:%S%z" | sed 's/\(..\)$/:\1/'
-    else
-        # Linux (GNU date) - use -u flag for UTC
-        date -u -Iseconds
+    # Try GNU date first (works on Linux and macOS with Homebrew coreutils)
+    local result
+    if result=$(date -u -Iseconds 2>/dev/null) && [[ -n "$result" ]]; then
+        echo "$result"
+        return
     fi
+    # Fallback to BSD date (native macOS) - add colon to timezone offset
+    date -u +"%Y-%m-%dT%H:%M:%S%z" | sed 's/\(..\)$/:\1/'
 }
 
 # Get time component (HH:MM:SS) for one hour from now
 # Returns: HH:MM:SS format
+# Uses capability detection instead of uname to handle macOS with Homebrew coreutils
 get_next_hour_time() {
-    local os_type
-    os_type=$(uname)
-
-    if [[ "$os_type" == "Darwin" ]]; then
-        # macOS (BSD date) - use -v flag for date arithmetic
-        date -v+1H '+%H:%M:%S'
-    else
-        # Linux (GNU date) - use -d flag for date arithmetic
-        date -d '+1 hour' '+%H:%M:%S'
+    # Try GNU date first (works on Linux and macOS with Homebrew coreutils)
+    if date -d '+1 hour' '+%H:%M:%S' 2>/dev/null; then
+        return
     fi
+    # Fallback to BSD date (native macOS)
+    if date -v+1H '+%H:%M:%S' 2>/dev/null; then
+        return
+    fi
+    # Ultimate fallback - compute using epoch arithmetic
+    local future_epoch=$(($(date +%s) + 3600))
+    date -r "$future_epoch" '+%H:%M:%S' 2>/dev/null || date '+%H:%M:%S'
 }
 
 # Get current timestamp in a basic format (fallback)