Hello, my DM gave my character a special longsword. I'm trying to create this longsword as a homebrew item in dndbeyond and having some difficulty. Specifically I'm running into three issues when using the longsword during gameplay:

1. The longsword is magic. It is a +1 magic longsword in the daytime and +2 at night. Should I just make a daytime version and nighttime version since this is pretty hard to do otherwise?
2. The longsword deals an extra damage die. So instead of 1d8+modifiers, it should be 2d8+modifiers. I am not sure how to do this in homebrew. I tried adding a modifier of [modifier type] of damage, [modifier subtype] of melee weapon attacks, [ability score] of STR, [dice count] of 2, [die type] of d8.
3. The longsword requires attunement to enable the magical effects (+1 daytime/+2 nighttime and the extra damage die). I am not sure how to do this as when I check the "attunement", the damage goes from 1d8+modifiers to 1d8+modifiers+5 instead of 2d8+modifiers. This could be tied to issue 2 above; maybe I didn't write the modifier correctly?

Can anyone help me resolve the 3 points above in homebrew? Or is it impossible :(

I'm going to answer this in a bit of an unorthodox order, tackling each part separately:

2. The longsword deals an extra damage die. So instead of 1d8+modifiers, it should be 2d8+modifiers. I am not sure how to do this in homebrew. I tried adding a modifier of [modifier type] of damage, [modifier subtype] of melee weapon attacks, [ability score] of STR, [dice count] of 2, [die type] of d8.

 
Try adding a modifier to the weapon as follows:
     • Modifier Type:  Damage
     • Modifier Subtype:  Slashing
     • Dice Count:  1
     • Die Type:  d8
     ... and check the box at the bottom that says "REQUIRES ATTUNEMENT".

3. The longsword requires attunement to enable the magical effects (+1 daytime/+2 nighttime and the extra damage die). I am not sure how to do this as when I check the "attunement", the damage goes from 1d8+modifiers to 1d8+modifiers+5 instead of 2d8+modifiers. This could be tied to issue 2 above; maybe I didn't write the modifier correctly?

 
There are 3 places you'll need to indicate that the weapon requires attunement, as detailed below: 
     1.  In the BASIC INFORMATION section, on the right-hand side you should see a checkbox labeled "REQUIRES ATTUNEMENT".  Make sure to check this box, then SAVE CHANGES.
     2.  There should be 2 modifiers for this weapon:
          a.  The first modifier should be 
                • Modifier Type:  Bonus
                • Modifier Subtype:  Magic
                • Fixed Value:  1
                • Check the box at the bottom that says "REQUIRES ATTUNEMENT", then SAVE
          b.  The second modifier should be 
                • Modifier Type:  Damage
                • Modifier Subtype:  Slashing
                • Dice Count:  1
                • Die Type:  d8
                • Check the box at the bottom that says "REQUIRES ATTUNEMENT", then SAVE

1. The longsword is magic. It is a +1 magic longsword in the daytime and +2 at night. Should I just make a daytime version and nighttime version since this is pretty hard to do otherwise?

 
Yes, since (to the best of my knowledge) there is no way to make the "Bonus - Magic" modifier (the modifier that adds the +1 or +2 to the weapon's attack and damage rolls) flexible/adjustable on the fly.  I'd just make one version of the weapon using the guidelines I've offered above, and append something like "(Daytime)" or "(Daybreak)" to the weapon's name, and then for the "Bonus - Magic" modifier, set the "FIXED VALUE" to 1.

Then, create a copy of the same weapon, appending something like "(Nighttime)" or "(Nightfall)" to the weapon's name, and then change the "FIXED VALUE" setting for the "Bonus - Magic" modifier to 2. 

Have both versions (Daytime and Nighttime) in your character's inventory, and depending on the time of day in-game, simply change which version of the weapon your character has equipped & attuned as would be appropriate for the situation.  

An addendum:

If you really want to get fancy with it, you can do this:

For the first weapon — for clarity, we'll call it "Magic Longsword (Daybreak)" — add one additional modifier:
          • Modifier Type:  Set
          • Modifier Subtype:  Attunement Slots
          • Fixed Value:  4
          • Check the box at the bottom that says "REQUIRES ATTUNEMENT", then SAVE


Then, for the second weapon — for clarity, we'll call it "Magic Longsword (Nightfall)" — make the following change:
          • In the ADDITIONAL INFORMATION section, find the empty field labeled "WEIGHT".  Enter this value:  0.00   
          • Click on SAVE CHANGES 


By doing both of these things, you can now have both the Day and Night versions of the weapon equipped and attuned at the same time, and you don't end up functionally losing any attunement slots due to the extra attunement slot granted by the modifier added to the "Daybreak" version.  Likewise, setting the weight of the "Nightfall" version of the weapon to zero makes it so that your total encumbrance is not affected by having both equipped at the same time.

You can't add or change damage dice on homebrew weapons. The only weapons that have 2d8 as their damage dice are all firearms:

• Blunderbuss (Exandria)
• Hand Mortar (Exandria)
• Revolver
• Rifle, Automatic
• Shotgun