Shiva Purana
चत्वारि तु सहस्राणि वर्षाणां तत्कृतं युगम् ॥ तस्य तावच्छतीसंध्या संध्यांशश्च तथाविधः ॥ १३ ॥
Four thousand years constitute the Kṛtayuga. The period of four hundred years constitutes the intervening junction and a hundred year period constitutes Sandhyāṃśa (a subdivision of the junction).
english translation
catvAri tu sahasrANi varSANAM tatkRtaM yugam ॥ tasya tAvacchatIsaMdhyA saMdhyAMzazca tathAvidhaH ॥ 13 ॥
hk transliteration by Sanscript