install: add time check and warning

Clock skew can cause problems with SSL and certificates. Check the UPS clock and log a warning for the user if the UPS clock is more than 1 hour different than the clock of the system this tool is running on

see: https://github.com/gregtwallace/apc-p15-tool/issues/11#issuecomment-2609010943

pkg/apcssh/cmd_gettime.go

@@ -0,0 +1,56 @@
+package apcssh
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+	"time"
+)
+
+// GetTime sends the APC `system` command and then attempts to parse the
+// response to determine the UPS current date/time.
+func (cli *Client) GetTime() (time.Time, error) {
+	result, err := cli.cmd("date")
+	if err != nil {
+		return time.Time{}, fmt.Errorf("apcssh: failed to get time (%s)", err)
+	} else if !strings.EqualFold(result.code, "e000") {
+		return time.Time{}, fmt.Errorf("apcssh: failed to get time (%s: %s)", result.code, result.codeText)
+	}
+
+	// capture each portion of the date information
+	regex := regexp.MustCompile(`Date:\s*(\S*)\s*[\r\n]Time:\s*(\S*)\s*[\r\n]Format:\s*(\S*)\s*[\r\n]Time Zone:\s*(\S*)\s*[\r\n]?`)
+	datePieces := regex.FindStringSubmatch(result.resultText)
+	if len(datePieces) != 5 {
+		return time.Time{}, fmt.Errorf("apcssh: failed to get time (length of datetime value pieces was %d (expected: 5))", len(datePieces))
+	}
+	dateVal := datePieces[1]
+	timeVal := datePieces[2]
+	formatUPSVal := datePieces[3]
+	timeZoneVal := datePieces[4]
+
+	// known APC UPS format strings
+	dateFormatVal := ""
+	switch formatUPSVal {
+	case "mm/dd/yyyy":
+		dateFormatVal = "01/02/2006"
+	case "dd.mm.yyyy":
+		dateFormatVal = "02.01.2006"
+	case "mmm-dd-yy":
+		dateFormatVal = "Jan-02-06"
+	case "dd-mmm-yy":
+		dateFormatVal = "02-Jan-06"
+	case "yyyy-mm-dd":
+		dateFormatVal = "2006-01-02"
+
+	default:
+		return time.Time{}, fmt.Errorf("apcssh: failed to get time (ups returned unknown format string (%s)", formatUPSVal)
+	}
+
+	// convert to time.Time
+	t, err := time.Parse(dateFormatVal+" 15:04:05 -07:00", dateVal+" "+timeVal+" "+timeZoneVal)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("apcssh: failed to get time (time parse failed: %s)", err)
+	}
+
+	return t, nil
+}