calendar: change the behavior of 1 months time+

calendar: change the behavior of 1 months time+

Before, it would create an invalid date:

  IN: scratchpad 2021 1 30 <date> 1 months time+ .
  T{ timestamp
      { year 2021 }
      { month 2 }
      { day 30 }
      { gmt-offset T{ duration { hour -6 } } }
  }

After, it sets it to the last day of the month:

  IN: scratchpad 2021 1 30 <date> 1 months time+ .
  T{ timestamp
      { year 2021 }
      { month 2 }
      { day 28 }
      { gmt-offset T{ duration { hour -8 } } }
  }