Calla Lilies are only hardy in zones 8 and warmer. In those warmer regions Callas bloom anywhere from 3 to 8 weeks during late spring to early summer. The variation in bloom time is influenced by temperature and light as well as varietal differences. I think it would be best to incorporate your Calla Lilies into a larger Summer Pot. Any warm season plant from Coleus to Petunias to Pelargoniums will work. Note that white and blue flowers are easiest to mix with other colors, especially unusual colors. There is no reason you can't dig a bit of the Creeping Jenny to plant with the Callas where it can trail over the side of the pot.

If you want to incorporate Callas into your garden beds, I think you would do better to plant the rhizomes in pots up to a month before the average last frost date. Place them in a sunny or at least brightly lit room where the soil temperature will be at least 65ºF. Once the threat of frost is past, you can dig a hole in a bed and plant the entire pot so that the soil level inside is the same as that outside the pot. The bulbs should bloom in 12 to 16 weeks.

During the growing season Calla Lilies need regular watering. If they don't get enough water the foliage may begin to yellow and the bulbs will fail to bloom. They also prefer sun, though they will benefit from afternoon shade in hot summer areas. Not getting enough sun can also cause failure to bloom. Callas bloom better with a low nitrogen high phosphorus fertilizer. When the flowers turn green and roll into tubes you should deadhead them, which will send more energy to the rhizomes for next year's bloom.

Dig up the pots in the fall. Water them less so the plants dry out. Cut off the dead tops. Remove the rhizomes and brush off any dirt, but do not wash them as this can lead to rot. Allow the bulbs to cure for 3 to 5 days then store them in a paper bag or wrapped in newspaper, or in slightly dampened peat moss. Store the bulbs in a dark, dry place that stays between 50º and 60ºF.

actually the very hardiest callas are a solid zone 7 hardy in the right spot

but those are mostly white ones like Zantedeschia aethiopica

I just found one in my overgrown cilantro plot...

Remove the rhizomes and brush off any dirt, but do not wash them as this can lead to rot. Allow the bulbs to cure for 3 to 5 days then store them in a paper bag or wrapped in newspaper, or in slightly dampened peat moss.

For clarification, does this mean that I can't just move the pot to my greenhouse?

I have several other plants that I keep in pots for that reason (Blue Java banana tree, cordyline, asparagus fern, dracaena grasses, echeveria, and some stonecrop), so it wouldn't be a big deal to keep it in a pot and move it. Digging it up and curing it would be a pain, though.