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From the very detailed man page:
Example 14. Show parsing of timespans
$ systemd-analyze timespan 1s 300s '1year 0.000001s'
Original: 1s
μs: 1000000
Human: 1s
Original: 300s
μs: 300000000
Human: 5min
Original: 1year 0.000001s
μs: 31557600000001
Human: 1y 1us